I was able to do this in forge in halo 3 (in sandbox) so I decided to try it in forge world. Watch in HD for best quality. YouTube - Halo Reach - Never Before Seen Forge Glitch! Map Download: Forge Glitch
Been there done that. Get the same thing when by yourself by just holding an object with you between it and the wall then moving it closer and loser until it eventually pushes you through. Or, even simpler just phase a teleporter through. Done and done. Not enough space back there or anywhere else in there for a map though. part of a map sure but only teleporters to get you in or out so highly camp-able.