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.