After our discussion in the Activiti Forum (http://forums.activiti.org/content/error-activity-doesnt-accept-signals-when-testing) , I made a unit test case and created that Jira as asked.
The process (located in src/main/resources/activiti/diagrams/test/MyProcess.bpmn) is very simple, it takes a long and add 2. After that, a user task is used to choose the right of the exclusive task and depending of that choice, it multiply or divide by 2 that long.
The test (located in src/test/java/com/foobar/test/testProcess/TestProcessTest.java) launch that process 500 times.
To launch the test, fill in src/main/resources/META-INF/configuration/database.properties with your database access.
Launch it with Maven with the goal : -Ptest -Dtest=TestProcessTest clean test (I'm launching it with Hudson)
The error occurs randomly so launch it again if it doesn't launch exception.
Don't hesitate to ask some details if needed