Minor Description
If timed just right, it seems like it is possible to add a group as a member of another group at about the same time when the first group is being deleted. But I think that can only happen if the membership being added is being added in a disabled state. Otherwise one of the transactions would fail (which would be good I think).
Bad membership finder should handle this case. Also, deleting the second group should not fail (it should delete the bad membership too).