An extra Warthog
Posted July 21st 2006 or earlier, expanded later
You can get an extra Warthog near the start of the level. Do a descent trick off the first bridge and back the hog into the indoor passage, all the way down the long middle section. Dismount and go up in the lift after bringing it down - be careful not to waltz into the empty lift shaft! Go through the room and into the passage beyond, far enough to trigger enemies in the room. You can go far enough to get a checkpoint too if you like, but it's not necessary. Now resume normal play, coming back down. When you get back to the hog, mount up and drive into battle. You'll find that the game has created a new hog outside, placed in the usual overturned position.
Note also that in contrast to the new hog (the usual hog), the original one you're driving is Marine-retaining, by which I mean that Marines will not leave it of their own accord. That can be useful! You'll be able to leave the hog and get continued chain-gun support.
I mentioned backing the hog right down the long section, but actually you just need to make sure it's further back than the loading point you'll be coming back across later. The critical thing is to be in the hog as you cross that loading point (at the end of the middle black stripe in that section), otherwise the game will remove it, effectively moving it outside into the overturned position.
What's going on?
If you're wondering why this trick works, here's my understanding. Firstly, the reason you went up to trigger enemies in the second room is because that's essential for being able to trigger the battle outside. Now let's consider what happens when you've come back down. Fractionally after you cross the loading point to head outside into the battle, that's when the game normally moves the original hog to the overturned position. Actually I suspect it removes the original hog entirely and creates a new one for the overturned position (the original is Marine-retaining while the overturned one isn't), but it amounts to much the same and it's obviously meant to be "the same hog". By being in the original at this time, you stop the game from taking it. Apparently it has a hands-off policy on things you're using; possibly some sort of safeguard, even though it really wasn't expecting you to be driving it here. However, it still creates a new one outside (as usual, I believe), thus you end up with two.
Fancy some extra Marines too?
With additional work you can also get some extra Marines if you like. See Two or three extra Marines and a hog (which includes a history section mentioning how I found out about getting an extra Warthog), and Seven extra Marines and a hog. With those methods too, the original hog will likewise be Marine-retaining. It's like that the moment it's dropped, and the game never changes it.