Running afterCompletion from reaper thread can get ConcurrentModificationException ---------------------------------------------------------------------------------- Key: JBTM-3021 URL: https://issues.jboss.org/browse/JBTM-3021 Project: JBoss Transaction Manager Issue Type: Bug Components: JTA Reporter: Amos Feng Assignee: Tom Jenkinson Fix For: 5.8.2.Final The way the locking works _syncLock and _synchs are used at different times and so it can be that _syncs is not locked and the .iterator() .next() might get CoModEx