There are several concurrency related exceptions being raised when using multiple threads, entry-points and sliding windows in 5.4.x+.

E.g.:

org.drools.RuntimeDroolsException: Unexpected exception executing action org.drools.reteoo.PropagationQueuingNode$PropagateAction@4de8b671
at org.drools.common.AbstractWorkingMemory.executeQueuedActions(AbstractWorkingMemory.java:979)
at org.drools.common.DefaultAgenda.fireUntilHalt(DefaultAgenda.java:1423)
at org.drools.common.AbstractWorkingMemory.fireUntilHalt(AbstractWorkingMemory.java:755)
at org.drools.common.AbstractWorkingMemory.fireUntilHalt(AbstractWorkingMemory.java:731)
at org.drools.impl.StatefulKnowledgeSessionImpl.fireUntilHalt(StatefulKnowledgeSessionImpl.java:247)
at org.drools.integrationtests.MultithreadTest$4.call(MultithreadTest.java:169)
at org.drools.integrationtests.MultithreadTest$4.call(MultithreadTest.java:1)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:679)
Caused by: java.util.NoSuchElementException
at java.util.LinkedList.remove(LinkedList.java:805)
at java.util.LinkedList.removeFirst(LinkedList.java:151)
at org.drools.common.PropagationContextImpl.evaluateActionQueue(PropagationContextImpl.java:344)
at org.drools.reteoo.PropagationQueuingNode$AssertAction.execute(PropagationQueuingNode.java:404)
at org.drools.reteoo.PropagationQueuingNode.propagateActions(PropagationQueuingNode.java:280)
at org.drools.reteoo.PropagationQueuingNode$PropagateAction.execute(PropagationQueuingNode.java:563)
at org.drools.common.AbstractWorkingMemory.executeQueuedActions(AbstractWorkingMemory.java:977)
... 14 more