Here's my contribution to the "tell me a story about a critical failure in one of your game sessions" story pool:
In a 3.5 game, I had a player running a half-orc barbarian who was destined to one day become the ruler of a small kingdom. Not wanting his kingdom run by a "gods-be-damned orc," a nobleman from that kingdom hired a doppelganger sorcerer/assassin to slay him with a magic dagger that would imprison his soul in its hilt, preventing him from being resurrected. The doppelganger took on the form of the head of the Adventurers Guild and brought the half-orc in to a one-on-one meeting, under the pretext that there had been an accusation made against him that needed to be addressed.
Once they were alone in the Guildmaster's office, the doppelganger activated spell-tokens that arcane locked the door to the room and made it such that no noise would emanate from the room, as the half-orc's dwarven bodyguard was standing just outside, keeping guard. Then, while the half-orc sat and read the complaint (the player had only recently spent the skill points to allow his barbarian to learn to read and write), the doppelganger took out the magic dagger and prepared for a death attack. He hit, but didn't do enough damage to slay the half-orc and imprison his soul - and now he had an enraged barbarian to deal with instead of one preoccupied with reading a trumped-up complaint. To stall for time, he cast an invisibility spell on himself and backed away.
The half-orc was bleeding from a stab wound, locked in an office with an invisible assassin, and had no weapons on him at the moment. So the player asked if he could lift the desk and throw it the length of the room, hopefully hitting the invisible assassin and pinning him in place. I told him to go for it, well aware the barbarian was easily strong enough to do so but I'd be adding penalties for a non-standard weapon and the overall weight of his "improvised weapon" - and then the player proceeded to roll a natural 1.
I took that situation as a particularly appropriate time for a critical failure (given the room size, just about anything else would have succeeded - there wasn't much chance the desk would miss) and had him slip on a piece of paper (the false complaint) he had been unaware he'd been standing on, which upended the desk right on top of him, pinning him in place instead of the doppelganger.
It all worked out in the end, though: he managed to extricate himself while the doppelganger was still lining up his next death strike (which takes three rounds) and tried again, this time knocking over the doppelganger with the hurled desk as originally intended. Eventually, it turned into a one-on-one fight between a (now visible, after his next attack) doppelganger sorcerer/assassin with a magic dagger against a raging half-orc barbarian armed with the splintered leg of the desk's chair. The doppelganger had his throat pierced by the chair leg and that was pretty much that. (And then the PCs hunted down the nobleman who had hired the hit in the first place....)
Johnathan