SFSB Passivation Thread: Caused by: java.lang.ClassNotFoundException

Issue

What does it mean when we see an error similar to:

2013-06-11 14:58:55,502 FATAL [org.jboss.serial.persister.RegularObjectPersister] (SFSB Passivation Thread - jboss.j2ee:ear=my-web-app.ear,jar=my-ejb.jar,name=SomeAction,service=EJB3) error
java.lang.reflect.InvocationTargetException
...
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:342)
    at org.jboss.ejb3.cache.simple.StatefulSessionFilePersistenceManager.passivateSession(StatefulSessionFilePersistenceManager.java:393)
    at org.jboss.ejb3.cache.simple.SimpleStatefulCache.passivate(SimpleStatefulCache.java:387)
    at org.jboss.ejb3.cache.simple.SimpleStatefulCache$SessionTimeoutTask.run(SimpleStatefulCache.java:305)
...
Caused by: java.lang.ClassNotFoundException: com.my.example.renderer.Renderer from BaseClassLoader@e47831d{vfsfile:/usr/local/jboss-eap-5.2/jboss-as/server/default/deploy/admin-console.war/}