Minecraft Half Dead Hack?

Hey guys, just had a question here.

While hackin it up on a games server, I came across something weird. You know when you die, your player turns red and falls over? Well, this guy was completely red, and half fallen over the entire game. It was obviously some sort of hack, because I killed him six of seven times and he respawned in the same manner. He was moving around, so it's different than some of the Godmode hacks I've seen. It didn't make him invincible or anything (or maybe it did, but my KillAura just bypassed it), but this would be really sweet for trolling, and even better while paired with a regeneration hack.

Several of my friends have seen this as well, but dont know what it is. If any of you know, please enlighten :3

As shown by these test pictures, it is only client sided:

Client side:


Server side:


I sometimes see this glitch in PvP too, but it must be some sort of problem on your end.