diff --git a/src/error.rs b/src/error.rs
index 8f24478..6f4de66 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -43,26 +43,31 @@ pub enum UnknownKeys {
 #[non_exhaustive]
 pub enum InferenceError {
     /// Expected and actual lengths of some array-shaped input differ.
+    #[non_exhaustive]
     MismatchedShape {
         kind: &'static str,
         expected: usize,
         got: usize,
     },
     /// An `Outcome` of the wrong variant was supplied for the requested inference.
+    #[non_exhaustive]
     WrongOutcomeKind {
         context: &'static str,
         expected: &'static str,
         got: &'static str,
     },
     /// A probability value is outside `[0, 1]`.
+    #[non_exhaustive]
     InvalidProbability { value: f64 },
     /// A scalar parameter is outside its valid range.
+    #[non_exhaustive]
     InvalidParameter { name: &'static str, value: f64 },
     /// An event contains tied teams, but the draw probability is zero.
     ///
     /// A zero draw probability asserts that draws cannot occur, so a tied
     /// result has no representable likelihood. Configure a positive `p_draw`
     /// (via `HistoryBuilder::p_draw` or `GameOptions::p_draw`) to admit ties.
+    #[non_exhaustive]
     TieWithoutDrawProbability { teams: (usize, usize) },
     /// The convergence sweep hit `max_iter` with the step still above
     /// `epsilon`.
@@ -77,6 +82,7 @@ pub enum InferenceError {
     /// oscillating rather than converging, in which case `alpha < 1.0` damps
     /// the within-game EP loop. [`History::converge_partial`](crate::History::converge_partial)
     /// returns the short fit instead when that is genuinely what is wanted.
+    #[non_exhaustive]
     NotConverged {
         iterations: usize,
         final_step: (f64, f64),
@@ -86,6 +92,7 @@ pub enum InferenceError {
     ///
     /// Indicates numerical breakdown; the resulting skills are meaningless
     /// and must not be treated as a converged estimate.
+    #[non_exhaustive]
     NonFiniteResult {
         context: &'static str,
         step: (f64, f64),
@@ -99,6 +106,7 @@ pub enum InferenceError {
     /// "last one wins" would make the result depend on iteration order.
     /// Declaring the same value repeatedly is fine and is the expected shape
     /// when a competitor's configuration is a property of the domain.
+    #[non_exhaustive]
     ConflictingCompetitorConfig {
         competitor: usize,
         field: &'static str,
@@ -113,6 +121,7 @@ pub enum InferenceError {
     /// `UnknownKey { team: 0, member: 0 }` learns nothing about *which* of its
     /// keys the history has not seen, and the natural handling — fall back to a
     /// neutral value — turns the whole thing into a plausible constant.
+    #[non_exhaustive]
     UnknownKey {
         team: usize,
         member: usize,
@@ -128,8 +137,10 @@ pub enum InferenceError {
     ///
     /// To change an existing competitor's configuration, supply it on an event
     /// through `Member`; that refits the whole history.
+    #[non_exhaustive]
     AlreadyRegistered { key: String },
     /// A prediction was given a team with no members.
+    #[non_exhaustive]
     EmptyTeam { team: usize },
     /// The prediction grid cannot resolve the narrowest feature in the matchup.
     ///
@@ -147,6 +158,7 @@ pub enum InferenceError {
     /// `predict_win_probabilities` answers the same matchup through adaptive
     /// quadrature and is accurate here; use it when only the per-team win
     /// probabilities are needed.
+    #[non_exhaustive]
     GridTooCoarse {
         /// Nodes required to resolve the narrowest feature.
         needed: usize,
@@ -154,8 +166,10 @@ pub enum InferenceError {
         max: usize,
     },
     /// A joint posterior was requested where one cannot be formed exactly.
+    #[non_exhaustive]
     JointUnavailable { reason: &'static str },
     /// Fewer than two teams were supplied to a prediction.
+    #[non_exhaustive]
     NotEnoughTeams { got: usize },
     /// The full outcome distribution was requested for too many teams.
     ///
@@ -165,6 +179,7 @@ pub enum InferenceError {
     /// enumerate on a caller's behalf; ask for individual rankings with
     /// `predict_ranking`, or for `predict_win_probabilities`, both of which
     /// stay cheap at any team count.
+    #[non_exhaustive]
     TooManyTeams { got: usize, max: usize },
 }
 
diff --git a/src/history.rs b/src/history.rs
index ebc3a63..1040842 100644
--- a/src/history.rs
+++ b/src/history.rs
@@ -963,7 +963,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
                     Some(skill) => skill,
                     None => match self.unknown_keys {
                         crate::UnknownKeys::Prior => Gaussian::from_ms(self.mu, self.sigma),
-                        _ => {
+                        crate::UnknownKeys::Reject => {
                             return Err(InferenceError::UnknownKey {
                                 team: team_idx,
                                 member: member_idx,
@@ -1220,7 +1220,7 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
                         mean += coefficient * self.mu;
                         *unseen.entry(format!("{key:?}")).or_insert(0.0) += coefficient;
                     }
-                    _ => {
+                    crate::UnknownKeys::Reject => {
                         return Err(InferenceError::UnknownKey {
                             team: 0,
                             member,
@@ -1482,7 +1482,8 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     ///
     /// `MismatchedShape` unless exactly two teams are supplied, `EmptyTeam`,
     /// `UnknownKey` under [`UnknownKeys::Reject`](crate::UnknownKeys::Reject),
-    /// and `JointUnavailable` if the latest slice holds ranked events.
+    /// and `JointUnavailable` if the history is empty or holds ranked events in
+    /// *any* slice — not merely the latest one.
     pub fn predict_margin(&self, teams: &[&[&K]]) -> Result<Gaussian, InferenceError>
     where
         K: std::fmt::Debug,
@@ -1538,8 +1539,6 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     /// elsewhere. See [`expected_information_gain`](crate::expected_information_gain)
     /// for the scale, the analytic `ln k` ceiling, and the cost.
     ///
-    /// # Errors
-    ///
     /// # Preconditions
     ///
     /// Every key must already be known to the history — that is, must have
@@ -1550,8 +1549,12 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     /// whole-team miss into a plausible constant, which is invisible to any
     /// test that does not assert on variation.
     ///
-    /// As [`History::member_skills`], plus `TooManyTeams` and anything
-    /// inference returns for a hypothetical outcome.
+    /// # Errors
+    ///
+    /// `NotEnoughTeams`, `EmptyTeam`, `UnknownKey` and `TooManyTeams` for the
+    /// shape of the request, `GridTooCoarse` when the performance sigmas are
+    /// too far apart to integrate on one grid, and anything inference returns
+    /// for a hypothetical outcome.
     pub fn expected_information_gain(&self, teams: &[&[&K]]) -> Result<f64, InferenceError>
     where
         K: std::fmt::Debug,
@@ -1652,6 +1655,8 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     /// # Errors
     ///
     /// `NotEnoughTeams`, `EmptyTeam`, `UnknownKey`, or `TooManyTeams`.
+    /// `GridTooCoarse` when the performance sigmas are too far apart to
+    /// integrate on one grid.
     pub fn predict_outcome(&self, teams: &[&[&K]]) -> Result<Prediction, InferenceError>
     where
         K: std::fmt::Debug,
@@ -1680,8 +1685,6 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     /// space, so it stays cheap at any team count — use it when you know which
     /// orderings you care about.
     ///
-    /// # Errors
-    ///
     /// # Preconditions
     ///
     /// Every key must already be known to the history — that is, must have
@@ -1692,8 +1695,11 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     /// whole-team miss into a plausible constant, which is invisible to any
     /// test that does not assert on variation.
     ///
+    /// # Errors
+    ///
     /// `NotEnoughTeams`, `EmptyTeam`, `UnknownKey`, or `MismatchedShape` if
-    /// `ranks` does not have one entry per team.
+    /// `ranks` does not have one entry per team. `GridTooCoarse` when the
+    /// performance sigmas are too far apart to integrate on one grid.
     pub fn predict_ranking(&self, teams: &[&[&K]], ranks: &[u32]) -> Result<f64, InferenceError>
     where
         K: std::fmt::Debug,
@@ -1739,6 +1745,10 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     /// `NonFiniteResult` if a sweep produces a NaN or infinite step. EP has
     /// broken down at that point and further iterations cannot recover, so the
     /// loop stops rather than reporting a NaN step as convergence.
+    ///
+    /// `InvalidParameter` if a competitor's drift model yields a negative or
+    /// non-finite variance — which also covers a custom [`Drift`]
+    /// implementation, the one case no constructor can check.
     pub fn converge(&mut self) -> Result<ConvergenceReport, InferenceError> {
         let report = self.converge_partial()?;
 
@@ -2289,10 +2299,18 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     /// # Errors
     ///
     /// - `MismatchedShape` if an event's outcome does not describe the same
-    ///   number of teams the event has, or if per-member weights do not match
-    ///   the team's membership.
-    /// - `InvalidParameter` if a per-event `score_sigma` override is not
-    ///   strictly positive.
+    ///   number of teams the event has. (Weights cannot mismatch here — they
+    ///   come one-per-`Member`; that check belongs to
+    ///   [`EventBuilder::weights`](crate::EventBuilder::weights), which builds
+    ///   them from a separate list.)
+    /// - `NotEnoughTeams` for an event with fewer than two teams, and
+    ///   `EmptyTeam` for a team with no members.
+    /// - `InvalidParameter` for a per-event `score_sigma` override that is not
+    ///   strictly positive, a non-finite score, rank or weight, or a
+    ///   `drift_scale` that is negative or non-finite.
+    /// - `ConflictingCompetitorConfig` if one competitor is given two different
+    ///   values for `prior` or `drift_scale`, whether within one batch or
+    ///   across batches.
     /// - `TieWithoutDrawProbability` if an event ties two teams while the
     ///   history's `p_draw` is zero. This includes `Outcome::winner(w, n)` for
     ///   `n >= 3`, which ties every loser.
@@ -2420,6 +2438,33 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
     }
 }
 
+/// Summarising rather than exhaustive.
+///
+/// A `History` owns every competitor's skill at every time slice, so a derived
+/// `Debug` would print the entire fit — megabytes for a real history, and
+/// useless in a log. This prints the shape instead. Same reasoning as `Joint`'s,
+/// which omits its `n^2` factorisation.
+///
+/// It exists at all because without it a consumer cannot `#[derive(Debug)]` on
+/// any struct holding a `History`, which is how both known consumers store it.
+impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> std::fmt::Debug
+    for History<T, D, O, K>
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("History")
+            .field("competitors", &self.keys.len())
+            .field("events", &self.size)
+            .field("time_slices", &self.time_slices.len())
+            .field("mu", &self.mu)
+            .field("sigma", &self.sigma)
+            .field("beta", &self.beta)
+            .field("p_draw", &self.p_draw)
+            .field("score_sigma", &self.score_sigma)
+            .field("unknown_keys", &self.unknown_keys)
+            .finish_non_exhaustive()
+    }
+}
+
 /// A factorised joint posterior, reusable across many queries.
 ///
 /// Built by [`History::joint`]. Every question the joint answers — the width of
diff --git a/src/outcome.rs b/src/outcome.rs
index 526f595..0546b9b 100644
--- a/src/outcome.rs
+++ b/src/outcome.rs
@@ -18,6 +18,7 @@ use smallvec::SmallVec;
 #[non_exhaustive]
 pub enum Outcome {
     Ranked(SmallVec<[u32; 4]>),
+    #[non_exhaustive]
     Scored {
         scores: SmallVec<[f64; 4]>,
         /// Per-event noise override. `None` means inherit
diff --git a/tests/convergence_strictness.rs b/tests/convergence_strictness.rs
index 03ab150..0910cf9 100644
--- a/tests/convergence_strictness.rs
+++ b/tests/convergence_strictness.rs
@@ -54,6 +54,7 @@ fn hitting_the_cap_is_an_error() {
             iterations,
             final_step,
             epsilon,
+            ..
         } => {
             assert_eq!(iterations, 1);
             assert!(
diff --git a/tests/degenerate_inputs.rs b/tests/degenerate_inputs.rs
index e3e4ef7..0510630 100644
--- a/tests/degenerate_inputs.rs
+++ b/tests/degenerate_inputs.rs
@@ -160,6 +160,7 @@ fn event_builder_rejects_a_weights_length_mismatch() {
                 kind: "weights",
                 expected: 1,
                 got: 2,
+                ..
             }
         ),
         "expected a weights MismatchedShape, got {err:?}"
diff --git a/tests/drift_scale.rs b/tests/drift_scale.rs
index 310a687..934fcb4 100644
--- a/tests/drift_scale.rs
+++ b/tests/drift_scale.rs
@@ -277,13 +277,11 @@ fn reject(scale: f64) -> InferenceError {
 
 #[test]
 fn negative_scale_is_rejected() {
-    assert_eq!(
+    assert!(matches!(
         reject(-1.0),
-        InferenceError::InvalidParameter {
-            name: "drift_scale",
-            value: -1.0
-        }
-    );
+        InferenceError::InvalidParameter { name: "drift_scale", value, .. }
+            if value == -1.0
+    ));
 }
 
 #[test]
diff --git a/tests/event_builder_members.rs b/tests/event_builder_members.rs
index f182ab0..cd098eb 100644
--- a/tests/event_builder_members.rs
+++ b/tests/event_builder_members.rs
@@ -135,7 +135,8 @@ fn weights_still_guards_a_members_team() {
             InferenceError::MismatchedShape {
                 kind: "weights",
                 expected: 2,
-                got: 1
+                got: 1,
+                ..
             }
         ),
         "{err:?}"
diff --git a/tests/game.rs b/tests/game.rs
index 5330523..6681990 100644
--- a/tests/game.rs
+++ b/tests/game.rs
@@ -155,7 +155,7 @@ mod malformed_games {
         let err = Game::<i64, _>::ranked(&[&[a]], Outcome::winner(0, 1), &GameOptions::default())
             .unwrap_err();
         assert!(
-            matches!(err, InferenceError::NotEnoughTeams { got: 1 }),
+            matches!(err, InferenceError::NotEnoughTeams { got: 1, .. }),
             "{err:?}"
         );
     }
@@ -173,7 +173,7 @@ mod malformed_games {
         )
         .unwrap_err();
         assert!(
-            matches!(err, InferenceError::NotEnoughTeams { got: 1 }),
+            matches!(err, InferenceError::NotEnoughTeams { got: 1, .. }),
             "{err:?}"
         );
     }
@@ -184,7 +184,7 @@ mod malformed_games {
             Game::<i64, ConstantDrift>::ranked(&[], Outcome::ranking([]), &GameOptions::default())
                 .unwrap_err();
         assert!(
-            matches!(err, InferenceError::NotEnoughTeams { got: 0 }),
+            matches!(err, InferenceError::NotEnoughTeams { got: 0, .. }),
             "{err:?}"
         );
     }
@@ -198,7 +198,7 @@ mod malformed_games {
             Game::<i64, _>::ranked(&[&[], &[a]], Outcome::winner(0, 2), &GameOptions::default())
                 .unwrap_err();
         assert!(
-            matches!(err, InferenceError::EmptyTeam { team: 0 }),
+            matches!(err, InferenceError::EmptyTeam { team: 0, .. }),
             "{err:?}"
         );
     }
diff --git a/tests/ingestion_shape.rs b/tests/ingestion_shape.rs
index cd43ab4..5c2e43f 100644
--- a/tests/ingestion_shape.rs
+++ b/tests/ingestion_shape.rs
@@ -40,7 +40,7 @@ fn a_one_team_event_is_an_error_not_a_panic() {
         }])
         .unwrap_err();
     assert!(
-        matches!(err, InferenceError::NotEnoughTeams { got: 1 }),
+        matches!(err, InferenceError::NotEnoughTeams { got: 1, .. }),
         "{err:?}"
     );
 }
@@ -56,7 +56,7 @@ fn a_zero_team_event_is_an_error() {
         }])
         .unwrap_err();
     assert!(
-        matches!(err, InferenceError::NotEnoughTeams { got: 0 }),
+        matches!(err, InferenceError::NotEnoughTeams { got: 0, .. }),
         "{err:?}"
     );
 }
@@ -75,7 +75,7 @@ fn an_empty_team_is_an_error_rather_than_a_free_win() {
         }])
         .unwrap_err();
     assert!(
-        matches!(err, InferenceError::EmptyTeam { team: 0 }),
+        matches!(err, InferenceError::EmptyTeam { team: 0, .. }),
         "{err:?}"
     );
     // Nothing was recorded, so the history is still empty.
@@ -93,7 +93,7 @@ fn an_empty_team_is_reported_by_position() {
         }])
         .unwrap_err();
     assert!(
-        matches!(err, InferenceError::EmptyTeam { team: 1 }),
+        matches!(err, InferenceError::EmptyTeam { team: 1, .. }),
         "{err:?}"
     );
 }
@@ -170,7 +170,7 @@ fn the_event_builder_inherits_the_shape_checks() {
     let mut h = history();
     let err = h.event(1).team(["a"]).winner(0).commit().unwrap_err();
     assert!(
-        matches!(err, InferenceError::NotEnoughTeams { got: 1 }),
+        matches!(err, InferenceError::NotEnoughTeams { got: 1, .. }),
         "{err:?}"
     );
 }
diff --git a/tests/non_finite_results.rs b/tests/non_finite_results.rs
index 26f7424..978b7b6 100644
--- a/tests/non_finite_results.rs
+++ b/tests/non_finite_results.rs
@@ -56,7 +56,7 @@ fn overflow_during_inference_is_reported_not_hidden() {
 
     for (name, sigma, beta, score_sigma, scores) in cases {
         match scored_fit(sigma, beta, score_sigma, scores) {
-            Err(InferenceError::NonFiniteResult { context, step }) => {
+            Err(InferenceError::NonFiniteResult { context, step, .. }) => {
                 assert_eq!(context, "History::converge", "{name}");
                 assert!(
                     !step.0.is_finite() || !step.1.is_finite(),
diff --git a/tests/predict_margin.rs b/tests/predict_margin.rs
index 95b70c5..c85866a 100644
--- a/tests/predict_margin.rs
+++ b/tests/predict_margin.rs
@@ -148,6 +148,6 @@ fn shape_errors_are_reported() {
     let empty: [&&str; 0] = [];
     assert!(matches!(
         h.predict_margin(&[&[&"veteran"], &empty]),
-        Err(InferenceError::EmptyTeam { team: 1 })
+        Err(InferenceError::EmptyTeam { team: 1, .. })
     ));
 }
diff --git a/tests/prediction.rs b/tests/prediction.rs
index d3595ea..5a0f792 100644
--- a/tests/prediction.rs
+++ b/tests/prediction.rs
@@ -20,13 +20,13 @@ fn unknown_keys_are_reported_not_silently_dropped() {
     let err = h
         .predict_outcome(&[&[&"a"], &[&"ghost"]])
         .expect_err("an unknown key must not yield a confident prediction");
-    assert_eq!(
-        err,
-        InferenceError::UnknownKey {
-            team: 1,
-            member: 0,
-            key: "\"ghost\"".to_owned(),
-        }
+    assert!(
+        matches!(
+            &err,
+            InferenceError::UnknownKey { team: 1, member: 0, key, .. }
+                if key == "\"ghost\""
+        ),
+        "{err:?}"
     );
 
     // Every prediction entry point, not just one.
@@ -42,13 +42,13 @@ fn unknown_keys_are_reported_not_silently_dropped() {
 fn an_entirely_unknown_team_is_an_error() {
     let h = history_with(&["a", "b"], 0.0);
     let err = h.predict_outcome(&[&[&"a"], &[&"x", &"y"]]).unwrap_err();
-    assert_eq!(
-        err,
-        InferenceError::UnknownKey {
-            team: 1,
-            member: 0,
-            key: "\"x\"".to_owned(),
-        }
+    assert!(
+        matches!(
+            &err,
+            InferenceError::UnknownKey { team: 1, member: 0, key, .. }
+                if key == "\"x\""
+        ),
+        "{err:?}"
     );
 }
 
@@ -56,18 +56,18 @@ fn an_entirely_unknown_team_is_an_error() {
 fn degenerate_team_shapes_are_errors_rather_than_panics() {
     let h = history_with(&["a", "b"], 0.0);
 
-    assert_eq!(
+    assert!(matches!(
         h.predict_outcome(&[&[&"a"]]).unwrap_err(),
-        InferenceError::NotEnoughTeams { got: 1 }
-    );
-    assert_eq!(
+        InferenceError::NotEnoughTeams { got: 1, .. }
+    ),);
+    assert!(matches!(
         h.predict_outcome(&[]).unwrap_err(),
-        InferenceError::NotEnoughTeams { got: 0 }
-    );
-    assert_eq!(
+        InferenceError::NotEnoughTeams { got: 0, .. }
+    ),);
+    assert!(matches!(
         h.predict_outcome(&[&[&"a"], &[]]).unwrap_err(),
-        InferenceError::EmptyTeam { team: 1 }
-    );
+        InferenceError::EmptyTeam { team: 1, .. }
+    ));
 }
 
 #[test]
@@ -93,13 +93,10 @@ fn the_outcome_space_is_capped_rather_than_hanging() {
     let refs: Vec<&[&&str]> = too_many.iter().map(Vec::as_slice).collect();
 
     let err = h.predict_outcome(&refs).unwrap_err();
-    assert_eq!(
+    assert!(matches!(
         err,
-        InferenceError::TooManyTeams {
-            got: 8,
-            max: MAX_PREDICTED_TEAMS
-        }
-    );
+        InferenceError::TooManyTeams { got: 8, max, .. } if max == MAX_PREDICTED_TEAMS
+    ));
 
     // The cheap paths stay available at any size.
     let wins = h.predict_win_probabilities(&refs).unwrap();
@@ -282,15 +279,12 @@ fn information_gain_respects_the_entropy_ceiling() {
 #[test]
 fn information_gain_reports_unknown_keys() {
     let h = history_with(&["a", "b"], 0.0);
-    assert_eq!(
-        h.expected_information_gain(&[&[&"a"], &[&"ghost"]])
+    assert!(matches!(
+        &h.expected_information_gain(&[&[&"a"], &[&"ghost"]])
             .unwrap_err(),
-        InferenceError::UnknownKey {
-            team: 1,
-            member: 0,
-            key: "\"ghost\"".to_owned(),
-        }
-    );
+        InferenceError::UnknownKey { team: 1, member: 0, key, .. }
+            if key == "\"ghost\""
+    ));
 }
 
 /// A draw-enabled history has three outcomes to weigh rather than two, so the
diff --git a/tests/prediction_bounds.rs b/tests/prediction_bounds.rs
index 0b9267e..307f8c6 100644
--- a/tests/prediction_bounds.rs
+++ b/tests/prediction_bounds.rs
@@ -154,7 +154,7 @@ fn the_known_ceiling_violation_no_longer_answers_wrongly() {
             gain <= 2.0_f64.ln() + 1e-9,
             "returned {gain}, over the ln 2 ceiling"
         ),
-        Err(InferenceError::GridTooCoarse { needed, max }) => {
+        Err(InferenceError::GridTooCoarse { needed, max, .. }) => {
             assert!(needed > max, "needed {needed} should exceed max {max}");
         }
         Err(e) => panic!("unexpected error {e:?}"),