This glitch occours when, at any time during a single player match of any type in Halo: Reach, a new player locally logs in and initiates split screen. If this new player has a Sangheili archetype selected and the game mode permits its use, then this glitch may activate.
This glitch causes an interesting cosmetic effect. To anyone else viewing, your character appears to be a completely default Spartan, but in first person the character's arms and legs appear to be that of a Sangheili Minor without the armor on the wrists. There do not seem to be any adverse effects on gameplay.
The only known cause is to sign in to a game already in session.
The only way to fix this glitch is to die or respawn, and there is no way to prevent this from happening.