diff --git a/docs/_source/api/derivations.rst b/docs/_source/api/derivations.rst
index 6637daf9..4d6f5b2f 100644
--- a/docs/_source/api/derivations.rst
+++ b/docs/_source/api/derivations.rst
@@ -10,6 +10,10 @@ means that they must depend on the same factors in the same order, have
 the same window width, same window stride, and the same starting
 trial. However, :class:`.ElseLevel` is compatible with any derivation,
 as long as other levels in the same factor have compatible derivations.
+For every combination of levels in the factors that a derived factor
+depends on, there must be exactly one matching derived level, taking
+into account that a level created with :class:`.ElseLevel` matches a
+combination that is not matched by other levels.
 
 .. class:: sweetpea.Derivation()
 
diff --git a/example_programs/Paper.py b/example_programs/Paper.py
index 87f9e002..8c076263 100644
--- a/example_programs/Paper.py
+++ b/example_programs/Paper.py
@@ -32,7 +32,7 @@ def one_diff(colors, words):
         return words[0] == words[-1]
 
 def both_diff(colors, words):
-    return not one_diff(colors, words)
+    return (colors[0] != colors[-1]) and (words[0] != words[-1])
 
 one = DerivedLevel("one", Transition(one_diff, [color, word]))
 both = DerivedLevel("both", Transition(both_diff, [color, word]))
diff --git a/sweetpea/_internal/derivation_processor.py b/sweetpea/_internal/derivation_processor.py
index 35542267..6578a4ae 100644
--- a/sweetpea/_internal/derivation_processor.py
+++ b/sweetpea/_internal/derivation_processor.py
@@ -57,8 +57,9 @@ def generate_derivations(block: Block) -> List[Derivation]:
         accum = []
         for factor in derived_factors:
             according_level: Dict[Tuple[Any, ...], DerivedLevel] = {}
+            # every level must have the same cross product, so we can get it once:
+            cross_product: List[Tuple[Level, ...]] = factor.levels[0].get_dependent_cross_product()
             for level in factor.levels:
-                cross_product: List[Tuple[Level, ...]] = level.get_dependent_cross_product()
                 valid_tuples: List[Tuple[Level, ...]] = []
                 for level_tuple in cross_product:
                     args = [(level.name if not isinstance(level, BeforeStart) else None) for level in level_tuple]
@@ -98,6 +99,16 @@ def generate_derivations(block: Block) -> List[Derivation]:
                                                                        block.variables_per_trial())
                     level_index = block.first_variable_for_level(factor, level)
                     accum.append(Derivation(level_index, shifted_indices, factor))
+            # check that everything in the cross product is covered by some level
+            for level_tuple in cross_product:
+                if level_tuple not in according_level:
+                    in_crossing = block.factor_in_crossing(factor)
+                    maybe_crossing = "crossed " if in_crossing else ""
+                    args = [(level.name if not isinstance(level, BeforeStart) else None) for level in level_tuple]
+                    if level.window.width != 1:
+                        args = list(chunk_dict(args, level.window.width))  # type: ignore
+                    block.errors.add(f"No level in {maybe_crossing}factor"
+                                     f" '{factor.name}' has a precicate that matches '{args}'.")
         return accum
 
     @staticmethod
diff --git a/sweetpea/tests/test_derivation_processor.py b/sweetpea/tests/test_derivation_processor.py
index ad582661..952fb668 100644
--- a/sweetpea/tests/test_derivation_processor.py
+++ b/sweetpea/tests/test_derivation_processor.py
@@ -86,6 +86,28 @@ def test_generate_derivations_should_produce_warning_if_some_level_is_unreachabl
     block.show_errors()
     assert capsys.readouterr().out == "WARNING: No matches to the factor 'congruent?' predicate for level\n 'dum'.\n"
 
+def test_generate_derivations_should_produce_error_if_some_combination_is_uncovered(capsys):
+    local_con_factor = Factor("congruent?", [
+        DerivedLevel("con", WithinTrial(op.eq, [color, text])),
+        DerivedLevel("dum", WithinTrial(lambda c, t: c=='red' and t=='blue', [color, text]))
+    ])
+    block = CrossBlock([color, text, local_con_factor],
+                       [color, text],
+                       [Reify(local_con_factor)])
+    block.show_errors()
+    assert capsys.readouterr().out == "No level in factor 'congruent?' has a precicate that matches '['blue', 'red']'.\n"
+
+def test_generate_derivations_should_produce_error_if_some_transition_is_uncovered(capsys):
+    local_repeats_factor = Factor("repeats?", [
+        DerivedLevel("yes", Transition(lambda colors, texts: colors[0] == colors[-1] and texts[-1] == texts[0], [color, text])),
+        DerivedLevel("no",  Transition(lambda colors, texts: colors[0] != colors[-1] and texts[-1] != texts[0], [color, text]))
+    ])
+    block = CrossBlock([color, text, local_repeats_factor],
+                       [color, text],
+                       [Reify(local_repeats_factor)])
+    block.show_errors()
+    assert "No level in factor 'repeats?' has a precicate that matches '[{-1: 'red', 0: 'blue'}, {-1: 'red', 0: 'red'}]'.\n" in capsys.readouterr().out
+
 def test_generate_derivations_within_trial():
     assert DerivationProcessor.generate_derivations(blk) == [
         Derivation(4, [[0, 2], [1, 3]], con_factor),