Invincible Warthog

In the level, Outskirts, there is an invincible Warthog. Play through the level normally until you get into the long tunnel in which you have to drive a hog. When you go in there, go pass the first "Loading" message. When it is done, turn around and drive all the way back to the crashed Pelican near where you got the first Warthog. Continue past the Pelican. The invincible Warthog is on a platform on the right. If you came here before, then the Warthog will not show up. Jump up there and use the Rocket Launcher or drive into the little planters in front of the Warthog to get it off the platform. This Warthog can be damaged, and it's cowlings can be removed, but the Warthog can not be destroyed.

Another way to get an invincible Warthog is to go to any level that you can get a Warthog and have an NCO (Half-Jaw or Sergeant Johnson) get inside the Warthog. As long as they are inside, the Warthog is invincible. This will also work with Spectres and Scorpions.