Arbiter eye glitch
From Halopedia, the Halo wiki
The Arbiter eye glitch is a glitch that occurs during Halo 3's Campaign. The graphical error, which causes one of the Arbiter's eyes to turn completely transparent, occurs after cutscenes in various levels, including Sierra 117, Crow's Nest, and The Covenant.
At the very beginning of the level, Crow's Nest, stand on the Arbiter's left and look at his head. His left eye will disappear and reappear numerous times. You can see straight through his head when it disappears.
This happens to both of his eyes, but never at the same time. He will still behave as if he's looking at you with both eyes. Also, his eyes are meant to be orange with black slits instead of pure black.
This also happens:
- Near the end of the level Sierra 117.
- on the level The Covenant, shortly after the cutscene where the Thel 'Vadam kills the Prophet of Truth.