Arbiter eye glitch
From Halopedia, the Halo wiki
The Arbiter Eye Glitch is visual glitch in the Halo 3 level, Crow's Nest.
Walkthrough
At the very beginning of the level, walk to the left side of the Arbiter, his left eye will disappear and re-appear numerous times. You can see straight through his head when it has disappeared.
Interestingly, this only happens to his left eye, never his right eye. He still behave as if he's looking at you with both.