Xbox 360: Black ops - "Five"?

Ok, so me and my mate were having a little wonder around the Pentagon, just looking at some of the easter eggs, like the wonder weapons on the tables.. anyway, it came to round 7, and we finished that round and left one runner zombie alive for a bit. As I teleported to the war room (where you turn the defcons on) the zombie teleported there as did my mate, so my mate went down... I killed the zombies and tried to revive my mate, he stabbed me, my character fell down and game over appeared... What happened here, I know 100% that stabbin a teammate on zombies does nothing...