I was just burned by an issue in Play Framework's JPA support of orphanRemovals. The issue manifests as a "collection owner not associated with session" JPAException within Play Framework 1.2.x - it was supposed to be addressed in 1.2.5 with this patch, but I'm running 1.2.5 and am still running into it.
I didn't take the time to diagnose or file a report - for now I'm punting by restricting the use of 'orphanRemoval'.