MAC gun glitch
From Halopedia, the Halo wiki
During the Return to Sender chapter of the level, there is a metal object moving back and forth -- the loading arm of the MAC. When it moves back, a MAC shell is loaded into it, and when it moves forward, it loads the shell into the MAC. If you get in the way of the loading arm while it moves forward, you will be killed, but you will also drop through a hole and see the air lock shown in the Return to Sender cutscene.
It is also possible to get stuck inside the MAC's barrel. Simply stand in front of the loading arm and wait for it to push you into the barrel. When the MAC fires, you will be killed, but you will also be launched into space. Elites can also get trapped in the barrel, making it difficult, if not impossible, to finish the level.
Standing on the opposite side of the room, where the loading arm receives the MAC shells, will have a similar effect; that is, if you stand directly behind the loading arm when it pulls back, you will be crushed by an invisible barrier and sent flying. Sometimes you will be thrown out of the level and killed.