Frozen Elite

The Frozen Elite is a glitch which can be found on the Pillar of Autumn of Halo:CE and on Cortana on Halo 3. note that the glitches are different in all but name one is a live Elite freezing in place and another is a dead one with a glitched death animation causing his feet to be stuck in a wall and the phyisics engine to think he is on the ground.

Halo: Combat Evolved
On the level Pillar of Autumn, when you are going to the bridge and you run into an Elite, there are some Marines shooting at him. Lead him through the door toward the Marines and he will freeze. You can walk through him and the Marines won't shoot him, however, if you attack him, it will sound like you hit a solid body. Also on some other levels when doing co-op if one person kills an elite and the other person hits a checkpoint at the same time or less than a second after the elite will sometimes freeze while falling.

Halo 3
There is also a frozen Elite on the Cortana level in Halo 3. As you travel through the infested High Charity, you'll come to an Elite sticking out of the ground at an oblique angle. He is clearly dead, and can only be unfrozen by hitting or shooting him.