Frozen Elite

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. Interestingly, if you throw a plasma grenade (by devmode) at him, the grenade won't explode.

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 unfroze by hitting him.