It is realted to anomaly: http://opensource.atlassian.com/projects/hibernate/browse/HHH-1540
'When using custom event listeners, sometimes you mess with (and initialize) lazy collections by mistake, which causes CollectionEntry.postFlush(PersistentCollection) to throw an AssertionFailure'
It is questionable why AT ALL exception is raised in this case!
What is the most important: EJB3 spec is not prohibiting to check/read/ object involved in the event, so current implementation is agains EJB3 spec.
Events are often used for validation so it is quite probable that the lazy property (collection) has to be accessed - and when it is lazy and not initialized before the exception is raised. Why hibernate cannot treat it in nicer way and allow such actions.
One workaround is to not-use-lazy (very bad).
Second workaround: in the code outside flush (not in event) pre-initialize all lazy properties (in fact simulation of 'not-lazy).