diff --git a/experta/strategies.py b/experta/strategies.py
index 1b3d473..65c3a55 100644
--- a/experta/strategies.py
+++ b/experta/strategies.py
@@ -15,12 +15,17 @@ def get_key(self, activation):
     def _update_agenda(self, agenda, added, removed):
         for act in removed:
             act.key = self.get_key(act)
-            try:
-                idx = bisect.bisect_left(agenda.activations, act)
-                if agenda.activations[idx] == act:
-                    del agenda.activations[idx]
-            except IndexError:
-                pass
+            idx = bisect.bisect_left(agenda.activations, act)
+            for o in (0, 1, -1):
+                try:
+                    if agenda.activations[idx+o] == act:
+                        del agenda.activations[idx+o]
+                    else:
+                        continue
+                except IndexError:
+                    pass
+                else:
+                    break
 
         for act in added:
             act.key = self.get_key(act)
diff --git a/tests/unit/test_strategy_statemachine.py b/tests/unit/test_strategy_statemachine.py
new file mode 100644
index 0000000..6f6f289
--- /dev/null
+++ b/tests/unit/test_strategy_statemachine.py
@@ -0,0 +1,61 @@
+import pytest
+
+from hypothesis import strategies as st
+from hypothesis import assume, settings
+from hypothesis.database import DirectoryBasedExampleDatabase
+from hypothesis.stateful import Bundle, RuleBasedStateMachine, rule, invariant, consumes
+
+from experta.activation import Activation
+from experta.agenda import Agenda
+from experta.engine import KnowledgeEngine
+from experta.fact import Fact
+from experta.factlist import FactList
+from experta.rule import Rule
+from experta.strategies import DepthStrategy
+
+
+def get_rule_stm(strategy):
+    class StrategyStateMachine(RuleBasedStateMachine):
+        def __init__(self):
+            super(StrategyStateMachine, self).__init__()
+            self.model = set()
+            self.agenda = Agenda()
+            self.strategy = strategy()
+            self.fss = set()
+
+        activations = Bundle("activations")
+
+        @rule(target=activations,
+              r=st.integers(min_value=0),
+              fs=st.sets(st.integers(min_value=0), min_size=1))
+        def declare(self, r, fs):
+            assume((r, frozenset(fs)) not in self.fss)
+            self.fss.add((r, frozenset(fs)))
+
+            fs = [Fact(i, __factid__=i) for i in fs]
+            act = Activation(Rule(Fact(r)), facts=tuple(fs))
+
+            # Update agenda
+            self.strategy.update_agenda(self.agenda, [act], [])
+
+            # Update model
+            self.model |= set([act])
+
+            return act
+
+        @rule(act=consumes(activations))
+        def retract(self, act):
+            # Update agenda
+            self.strategy.update_agenda(self.agenda, [], [act])
+
+            # Update model
+            self.model -= set([act])
+
+        @invariant()
+        def values_agree(self):
+            assert set(self.agenda.activations) == self.model
+
+    return StrategyStateMachine
+
+
+test_depthstrategy_state_machine = get_rule_stm(DepthStrategy).TestCase