Halo 2: Glitch - Rooftop Glitch

When you first get on Earth, go through the first opening into the courtyard. Head over to the right side where the buildings are. Follow the wall until you see an alleyway on your right. Enter it and follow it left. Stand on top of the pile of dirt at the end of the alley and while facing the wall, do a jump/crouch to get on top of the buildings on the block. You'll be able to clip the stage at the very top.