This is expected behaviour. If you have concurrent threads changing the same node at the same time, one of the threads will fail. In pessimistic locking, the 2 threads are NOT ALLOWED to change nodes at the same time and one thread waits. This leads to poor concurrency. Optimistic locking allows both threads to access nodes at the same time and hopes that one thread will just read and not write. If simultaneous writes happen, you will naturally get a concurrent modification exception. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4020706#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...