OK
The level is Delta Halo (from Halo 2) when the Pelican comes to drop off the Scorpion just before the bridge you have to lower. I stand under the Scorpion to prevent it from dropping, when we (me and my brother) discover it's possible to pile vehicles on top of the Pelican by jumping them over this hill. Naturally the first thing we do is pile everything we can on and discover that the Pelcan sinks. This allows things other than Ghosts to be put on, which is exactly what we did. Even put a Wraith on.
So now my brother and a hillarious assortment of vehicles is on top of the Pelican and I step out from under the Scorpion (or out of as it had sank into me), and the Pelican begins to rise or tries to. What actually happened was it didn't move and started shaking from side to side. I hop on and my brother gets in the Wraith and pushes a bunch of Ghosts off. The dropship now rises more successfully, eventually reaching ideal height and taking off. We slide off and think "all that work for nothing."
But what's this? The Warthog and a Ghost stayed on, they had traction. Thus we repeat the endeavor and try to brace ourselves against a Warthog so that we may ride the Pelican and see where it goes. Failure yet again.
Alright we need some barrier to the sides as well as the back. A Wraith, it's got a perfect space in between two wings (a tank with wings, aren't we lucky). Repeat again, this time using the Wraith to brace ourselves. Failure once again, the Pelican shakes the Wraith off before it even leaves.
Repeat yet again, this time we steady the Wraith on the way up (with some careful driving) then jump out when the Pelican stabilizes for take off (and it was like take off, sudden enormous acceleration, just horizontal instead of verticle). Success! We ride the Pelican, giddy with exitement. And we were soon splattered by the Wraith as it and the Pelican passed through the invisble wall. It was worth it to see two Master Chiefs falling at the same speed, flailing in perfect unison, precisely parallel with each other as a pelican with a Wraith on it speed in to the distance.
On a related note we were doing that again and the Pelican was angled downward by the weight when it took off, hitting a rock. We pushed it with the Wraith to dislodge it, then it crashed promptly into another rock. Pushed it again and it flew straight into the cliff, for some reason it was turned nose downward bye the cliff (it crashed into it at an upward angle). And it kept crashing into little outcroppings as it angled ever more towards nose straight down. It eventually passed that angle and something clicked, it stopped rocketing forward and just started spinning in place. We approached causiously to inspect it and when I got near enough it said "Press X to flip Banshee."
I'm thinking, "no fracking way, we might get to fly this" (every Halo fan's dream since The Silent Cartographer). So I flip it, and it just doesn't behave right. The entire enormous craft just started spinning rapidly and moving away from me, morphing through geometry with the only solid part aparently being the cockpit. It eventually settles almost perfectly upsidedown so I flip the "Banshee" again. The same effect, only this time it starts rolling towards the cliff.
"No save it!" And my brother tried in vain, it start spinning faster and moved back up the slope a ways then tumbled towards the cliff again. We tried to get in it for quite awhile but it eventually fell over the abyss. We followed proclaiming that we'd save it and yelling "NOOOOOOOOOOOOOOOOOOOOOOOO!" as we fell to our deaths with it. The Pelican simply spun wildly on top of the water as we died.
Ahh, good times. And my mom still doesn't understand what all the screaming was for

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