I verified this issue, and I got the same ConcurrentModificationException. It only appeared with cascade save and the associationManagement enhancing property set.

I will not post a test case because I'm not interested in actually using Hibernate in actual projects. i am doing a comparison of ORMs, and as soon as its a bit more finished I will post an example where this exception occurs.

Tested with 5.1.0-FINAL.

Once you disable associationManagement enhancement, the bug is gone.

Enhancement in hibernate 5 seems to contain rather a lot of problems still.