When you play the map Graveyard, if you stay in the church for too long and the zombies build up, they can eventually get inside the church through the walls. There is a similar situation on many maps such as Volcano and Fortress whenever you play Cursed Survival where the undead spawn inside of a block and they can shoot you but you can't shoot them.

The glitches happen at:
1:20
1:40
1:45