Was messing around when this happened...haha. http://www.youtube.com/watch?v=YR33R_KrpKw
I guess a knife in the balls would make anyone moonwalk for eternity.
Oh and btw I know my accent sucks, I was just messing about, didn't expect this to happen.
It's faintly possible it's staged and he managed to turn the monster killable for the video, thinking it would have a death animation. Or it's just, as claimed, a weird bug.