In EventContextManager#onEventFinished, we have the following code:

for ( Entry<Class<?>, Object> state : states.entrySet() ) {
	if ( state.getValue() != session ) {
		onFinish( state.getKey(), state.getValue(), session );
	}
}

The issue is that an OperationCollector entry might be added to the states map in one of the onFinish() call (more specifically the one for the OperationsQueue), thus leading to a ConcurrentModificationException.