On Outskirts when you go through the tunnel all the way near the end of it, turn around and go back to where the crashed Pelican is on the beach. On a platform nearby, there will be a Warthog there. That Warthog is indestructible but you can still shoot off the bumpers and stuff. Pretty cool.
I did this on the original Xbox version of Halo 2 since I don't have an Xboner and the MCC. If that Warthog is still there on H2:A, that would be cool.
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The warthog is still there. I can confirm. You don't have to even go into the tunnel for it to spawn. Once you get the checkpoint by the entrance of the tunnel it spawns. You can then go back and use it for the entire tunnel (very helpful). I used this a couple of weeks ago for a legendary run.