Elite helmet glitch
From Halopedia, the Halo wiki
The Elite Helmet glitch is a glitch involving the helmets of various Elite corpses in Halo 3.
At the end of the level, Floodgate, when entering the crashed Flood ship, the player will find several dead Elites corpses lying around. Some of them have a strange helmet that appears to be a mixture of an Assault helmet and a Combat helmet. It appears that the game had attempted to render both helmets on the head of the Elite, and thus, both appear, overlapping one another. This is further proven by it having one Assault and one Combat shoulder. Some less informed players believe the glitched harness to be some kind of Sangheili version of Recon MJOLNIR armor, though this is not true.
Some speculate is that in the same crashed ship, bodies resembling Rtas 'Vadum (Half-Jaw) appear randomly which happen to be wearing Combat helmets, and that the Elite was wearing an Assault Armor helmet, and the game programming attempted to change the body into the Half-Jaw version.
Others speculate that the armor is actually an Easter Egg; the armor was supposedly intended to be included in the final game on SpecOps Elites, but never made it.
