Stuck inside a rock
Ever been stuck inside a rock or a tree trunk? In Halo it's possible, courtesy of what can only be considered a programming bug.
I got stuck inside a large rock when exiting the Warthog just as it was knocking up forcibly against the rock. As you've probably noticed, sometimes when do such a thing, you get instantaneously transported a few yards away from the collision spot. Well, this time, I seemed to get pushed through the rock's paper-thin surface, into the interior. Inside, the rock was completely invisible, except that I think it still cast a shadow. I could move around inside it, but there was no way past the invisible walls. Weird. Something similar once happened with a tree trunk.
Trying to do it
If you care to make enough attempts, you can probably get inside a rock yourself, by dismounting the Warthog whilst deliberately slamming it into the rock sideways. Maybe there's even a learnable technique of doing it, I don't know.
Stop press: much later (March 2005), I've found a completely different and fairly reliable way of getting inside a rock, namely in level 5's first ground battle. Jump to here for details.
Will any large rock do?
The actual rock I got stuck in was the one you'd normally pass at the right entrance when entering the rockslide area of level 2. I was probably trying to get out to do some sniping. But I also once got stuck inside one of the rocks up with the hill-top survivors in level 2. So, I'm guessing that any large rock will do. And if that's true, maybe it's possible to get inside one of the house-sized rocks in the rockslide area, giving you lots of room to move around in. Would the enemy be able to see you or shoot you from outside? Can you shoot them?
Falling through the ground
Possibly relating to the same bug, I've actually fallen through the ground in the rockslide area of level 2 a few times. Once, I dismounted the Warthog just as it was slamming up against the 'central rock' up at the plateau, and I got pushed through the ground somehow. For a few seconds I was falling through a mostly bluish space, with the ground above me. Very nice too. But not for the Marines, soon to be severely killed in my absence.