Halo 2 trickster extraordinaire Vernon "vshields ash" Shields has spent an enormous amount of time-- literally hundreds of hours-- exploring the depths of the infinite enemies scenario at the end of the level Regret, to try and see if anything different happens. So far, he's seen quite a bit. Through object overload, he's made Regret disappear (as well as other enemies, visual effects, and bits of furniture) as well as made over a hundred of the Prophet's chairs appear out of thin air. He's seen the Prophet stand up out of his chair, as well as utter bits of dialogue nobody else has heard. He's seen a silver grunt that fires several times faster than normal, as well as five different variations of the ending cutscene for Regret (one of which was parodied by Stuntmutt's latest One One Se7en).

All of this was achieved without modding.

This epic thread at High Impact Halo summarizes most of his observations, and also links to a video of these tricks made by Duelies from ash's advice. (Apparently there's a tape with more stuff like this on it, can't wait to see--Ed.)