I don't have a unit test but I could send you a trace if that helps you. The problem is happening in a transaction that is starting from a Message driven bean and calls several Session beans (all have transaction required attributes). So all the method calls participate in the same transaction. When I execute one message at a time everything works correctly but when I fill the queue with many messages (many concurrent transactions) the problem appears. In the Session beans the hibernate session usage is like... Session.open() ... Session.find() ... Session.close() When the operation is real-only, or Session.open() ... Session.find() ... Session.flush() Session.close() When the operation is read-write. When I use pessimistic locking everything works ok. The problem appears only when I use the optimistic locking cache. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3981230#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...