Hole in the Highway
From Halopedia, the Halo wiki
Summary
In the last part of the level Outskirts, you make your way through the enclosed highway. Partway through, a Scarab blasts a hole in the roof. The glitch is that you can climb on the debris and jump onto the untextured roof. Though Grenade Jumping helps, it is not neccessary. If you do it fast, you can see the Scarab walking away and then disappearing.
Also, if you drive a Ghost with it's propulsion activated, you can see that the Scarab isn't even firing the shot. It is actually a Plasma Rifle that spawned for the graphic destruction. This Plasma Rifle is otherwise known as the Scarab Gun. It is speculated that this Scarab Gun is obtainable in this level right when it spawns.
