Forge physics glitch
From Halopedia, the Halo wiki
The Forge Physics Glitch is a glitch in Halo: Reach's Forge. It allows you to get out of any map.
Tutorial
Start a Forge game over Xbox LIVE with at least two people. Have one person spawn a wall and set the physics to phased. Have another person stand on the place where they want to get under. Have the person with the wall push the person under the map with the wall, and then that person will fall under the map. If you go into monitor mode quick enough, you can survive and fly around.
