I know our team, Physics Remastered, has done some pretty amazing glitches, but this one takes the cake in Halo 3. I was finally able to get in to the Rec Room on Crow's Nest. This is with out a doubt the coolest room in Halo 3 I've been in and the video I made for it can teach you how to get in there as well. Enjoy!
http://www.youtube.com/watch?v=xh7yE9Fz-6Y
<3 Carnage A51