There was this guy... when you killed him, his body went rag-doll, but he was still alive. His body would then jump around the room several times. If you didn't kill him while he was "dead," he'd come back to life and kill you.
Another guy would run across a hall and not even look at you, but still kill you. He seemed to move faster, too.
I was playing 2 people on one Xbox 360. We do not believe we were having lag. Were these two people cheating?