In Mansion (Arcade)'s Last Stand and Dusk of the Dead, there's a very rare bug in which a zombie leaves the map's boundaries, rendering them unreachable and unkillable. This forces a restart as the next wave will not initiate unless all zombies are slain. If you have "Display Radar" ON, you can see the zombie in the Radar, usually in the U-Bend to the right side of the door.