Elite helmet glitch
From Halopedia, the Halo wiki
There is a glitch with the helmet of an Elite corpse in Halo 3.
At the end of the level Floodgate when entering the crashed Flood ship, a player will find several dead Elites lying around. Some of them have a strange cross-helmet that appears to be a blending of an Assault helmet and a Combat one. It appears that the game had attempted to render both helmets on the head of the Elite and thus both appear, overlapping one another.
A theory is that in the same crashed ship, bodies resembling Half-Jaw appear randomly which happen to be wearing Combat helmets, and that the Elite was wearing an Assault Armour helmet, and the game programming accidentally attempted to change the body into the Half-Jaw version. another theory is that it couldn't breath properly when the ship got took over by the flood so the elite took a part of another helmet and attached it to his
