Hole in the Highway
From Halopedia, the Halo wiki
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 roof. Though Grenade Jumping helps, it is not neccessary. If you do it fast, you can see the Scarab walking away and then disappearing.
If you can get through the level very fast and get to the part where the scarab blows a hole in the ceiling you will also notice very faintly a scarab gun in the blast. Someone managed to get it by grenade ghost jumping.
