My fault, my terrible fault. The exception is thrown by stateless session bean, not mdb. I reused same reference to that stateless session bean by multiple threads (I once did look up in initial context and then reused the reference). The threads then access same instance of entity manager which is wrong (entity manager is not thread safa object). Pavel View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4134369#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...