If you play Halo 4 multiplayer, chances are you've heard about the rumored headless Spartans that you can't kill. Maybe if you're unlucky you've actually come across these infamous Spartans.
One of the popular aspects of this rumor is that the headlessness is actually an extremely high level helmet unlock. But that would be kind of silly, wouldn't it? "Hey, you're so high level that earned the right to no longer have a weak spot." Yeah, right.
People around the web seem confused as to whether or not this is a glitch, but it's something that can done on modded Xboxes. Though the video here is but a match replay, if you happen to know someone who uses the cheat and visit their Halo Waypoint profile, their Spartan will look just like that. All muscular and headless, I mean.
Update: Though footage makes it seem otherwise, the hitboxes on Spartans are still intact, so you can still shoot people in the head. Microsoft's comment on banning the cheaters:
"We are aware of the issue and have already taken action to identify and ban the accounts of these modders and cheaters. We want players to know that while the mod makes a player's head and armor appear to be invisible, the hit boxes are still intact, so competing players can still defeat them as usual. While the problem was very isolated, we will continue to diligently identify and take action against all offenders. " – Microsoft spokesperson