yeh this has happened a few times, and i think has been reported before. It usually solves itself on the next map though. IMO, not really something to worry about. Just a good round for zombies i suppose.
Lol it happened to me once U G K was LMS so we were happy XD, it was bc and they just turned invincible. Nothing kills em not even backtap. Sadly ugk got a overkill on the arch campers -_- and it ended, it was funny for the LMS thought , he just ran like the wind.