Arbiter eye glitch
From Halopedia, the Halo wiki
The Arbiter Eye Glitch is a visual glitch in the Halo 3 level, Crow's Nest.
Walkthrough
At the very beginning of the level, Crow's Nest, walk to the left haunch and side of the Arbiter, his left eye will disappear and reappear numerous of times. You can see straight through his head when it has disappeared, for the brevity of the moment
Interestingly, this only happens to his left eye, never his right eye. He will still behave as if he's looking at you with both eyes. Also his eyes are supposed to be orange with black slits instead of pure black.