Let me start by saying this: Racing through a mine in a mine cart Temple of Doom-style should be mandatory for all D&D adventurers, at least once in their careers. Complete with jumps, hairpin corners taken on two wheels, monster attacks and collapsing tracks, it was possibly the most fun sequence of encounters I've ever had in a D&D game. In previous editions, I never would have considered taking this risk. I would have been afraid that my fragile character, especially at 4th level, would never have survived the jump or 40-foot drop off the top of the raised tracks. Instead, "the only thing missing," my character Deimos gleefully shouted to Mat Smith's character Garrot, "is fire! We need some explosions!"
Just to be perfectly clear, here: the player proposing this incredible venture in the first place (yours truly) is playing a character with Wisdom 8, to another player (Mr. Smith) playing a character with Intelligence 8. If that's not a match made for the Darwin awards, I don't know what is.
We also learned a lot about cart/monster physics in that adventure. For example, did you know that darkmantles have trouble grabbing your face when you whip by at 30 mph? In fact, if they make impact with a resilient surface—say a shield held out as a windbreak—they splatter like bugs?
But perhaps the best moment was when the other character on our little side adventure, the dragonborn warlord Abraxus played by Andrew Finch, saw the gap in the tracks ahead. Thinking in 3rd Edition terms, I'm sure, he decided the wise choice was to tumble out the back of the cart, taking just a few points of damage rather than risk "certain" death by mine cart. But after Garrot and Deimos made their jump—pulling an untrained Dungeoneering check out of their, well, you know—Abraxus could only marvel that as strange as it seemed, he made the wrong choice by falling out.