|This trick has been around for quite a while - but it's never been properly documented. The earliest reference I could find for it came from Commando_BOB, who posted this note on the HBO Forum on January 25, 2002:
we drove the warthog down the were they lock the door and just befor a grunt died he threw a grenade it fliped me out of the warthog and through the locked door and i was fine except for some sheild damage from the grenade.
A couple of weeks later, Nemesis 007 pinned down the technique on Sidewinder - check out the Sidewinder Sneak trick for details. It popped up again, in various forms, over the next couple of months, but it wasn't until April 5 that Bluespam got pictures. Frogblast kindly supplemented Bluespam's screenshots with video footage - with the added bonus of a not-fully-textured Gold Elite on the inside! (Check him out - he's got a matte finish!) A day later, xbill added his contribution to the mix - through the locked door, WITHOUT a partner to help push! (This is a direct application of the sidewinder sneak trick, as opposed to an assisted application.) He, too, discovered a braindead gold elite on the inside - apparently, the guy's not scripted to do anything until you unlock the door the RIGHT way (or activate the Cartographer).
The key to this trick is simple - you must be forced through the door by placing your body in a place already occupied (according to the physics engine) by something else - this can be done with a grenade (as per Commando_BOB), a slow-moving vehicle (as per Bluespam/Frogblast - be careful with this; if you leave too much space between the jeep and the MC, you'll simply die instead of being pushed through the door), or a fast-moving vehicle and the enter/exit animation (as per xbill). See where else you can apply this!