I have created a test case to illustrate this for both SimpleStatefulCache and StatefulTreeCache, NestedBeanUnitTestCase (cache and clusteredsession). Basically, if we do this:

bean.getXXX
sleep(10000); // to passivate
bean.getXXX; // to acitvate
sleep(10000); // to passivate again

This will produce:

2006-07-24 04:46:59,358 ERROR [org.jboss.ejb3.cache.simple.SimpleStatefulCache] problem passivation thread^M
javax.ejb.EJBException: Could not passivate; failed to save state^M
at org.jboss.ejb3.cache.simple.StatefulSessionFilePersistenceManager.passivateSession(StatefulSessionFilePersistenceManager.java:363)^M
at org.jboss.ejb3.cache.simple.SimpleStatefulCache.passivate(SimpleStatefulCache.java:196)^M
at org.jboss.ejb3.cache.simple.SimpleStatefulCache$SessionTimeoutTask.run(SimpleStatefulCache.java:129)^M
Caused by: java.lang.NullPointerException^M
at org.jboss.serial.util.StringUtil.calculateUTFSize(StringUtil.java:319)^M
at org.jboss.serial.util.StringUtil.saveString(StringUtil.java:63)^M
at org.jboss.serial.objectmetamodel.DataContainer$DataContainerDirectOutput.writeUTF(DataContainer.java:265)^M
at org.jboss.ejb3.stateful.StatefulBeanContextReference.writeExternal(StatefulBeanContextReference.java:65)^M
at org.jboss.serial.persister.ExternalizePersister.writeData(ExternalizePersister.java:58)^M
at org.jboss.serial.objectmetamodel.ObjectDescriptorFactory.describeObject(ObjectDescriptorFactory.java:275)^M
at org.jboss.serial.objectmetamodel.DataContainer$DataContainerDirectOutput.writeObject(DataContainer.java:197)^M
at org.jboss.serial.io.JBossObjectOutputStream.writeObjectOverride(JBossObjectOutputStream.java:181)^M
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:287)^M
at org.jboss.ejb3.cache.simple.StatefulSessionFilePersistenceManager.passivateSession(StatefulSessionFilePersistenceManager.java:347)^M
... 2 more^M

This is caused by a bug from ProxiedStatefuleBeanContext serialization where we dont serialiaze parentRef if it is not null.