Don’t know if this was on the original game (probably), but:
On “Halo”, when you get to the pod where you pick up the sniper rifle, kill off all the enemies, drive your Warthog down to & past the pod. Right before the outlet to the next area, there’s a huge boulder on the right. Drive your Warthog up in the left-side crevice and then pop out. What happened to me (somehow) is I fell beneath the map and died. I was intending on jumping out to get on top of the Warthog and check the top of the rock for… who knows (a skull, maybe? I didn’t look at the spoilers for locations, heh). Although I’ve now found the skulls on this mission, it’s another fun “walk through the wall” glitch like the one on “Library.”