The craziest thing I've ever seen in D&D actually is related to the black d20 from the My Favorite Dice entry.
It was a game of D&D 4E, I was DMing, and the party was running through a standard dungeon crawl. I think it was Treasure of Talon Pass, but I'm honestly not sure. Anyway, the party, which was only three PCs (a cleric, a wizard, and a fighter, I think), was up against a group of kobolds. It should have been a fairly easy fight; there weren't a lot of kobolds, but they had some cover. The kobolds won initiative, so the party had to weather some fire before getting to fight back. Now, it's important to note that I rolled in front of the players for this whole encounter. You'll see why.
The first kobold threw a spear at the wizard and rolled a crit. The wizard was immediately bloodied (he had pretty low HP). Okay, it's easy enough to deal with that; the cleric had plenty of healing to spare. The second kobold threw another spear at the wizard, with another crit. The wizard dropped to negative hit points and the players started to sweat a little. The third kobold stepped up to stab the fighter. Another crit and the fighter is just above bloodied (he'd been hit in an earlier fight and was a few hit points down already). The fourth and fifth both hit the cleric. Both of them rolled critical hits and the cleric went down.
Five critical hits in a row. The entire table was speechless. Five straight crits was unheard of. The fighter had no healing and no area attacks. He got wrecked.
After that, it was the players' turns, but they couldn't pull out of that five critical hit fiasco. The fighter fought on for a couple rounds, but the wizard and cleric bled out with some terrible death save rolls. The fighter got overwhelmed by the kobolds pretty rapidly after that. To top it all off, it was only the second time I'd DMed 4E.