Pat. St. of Archive Binge
I once had an Orpheus player who rolled several supers while trying to catch a fangirl going after the (haunted) teen heartthrob she was supposed to be protecting. Instead of simply tackling her, the PC simply held her arm out, clotheslined the screaming teen, and stunned her long enough to drag her out of the room.
Another critfail from a boss fight: It has been established that standing on or being pushed through a fire does 2d10+5 damage to the unfortunate character. The boss chamber happened to contain two solid walls of fire with the main area of the fight in between them and a bit of floor space on the other side. This wouldn't be a problem if it weren't for the fact that the boss in question had an ability that let him push people through said walls of fire. I personally thought this was a bit of a game-breaker so I didn't really let him use it until he somehow got pissed. After our mezzer hit him with what was functionally -3 to attack and damage, I decided to go for it. One critfail on the saving throw later, our mezzer is pinned to a wall on the other side of the flames, at -5 HP, and bleeding out.
Also from that session, and almost immediately following the above (I don't recall the exact sequence of events): Our monk saw this and entered a state of Tranquil Fury. He calmly dodged a goblin's attack of opportunity, walked straight up to the boss, readied a mighty blow... and knocked himself out.
Tangentially related to the thread...
In the French game Metal Adventures, the players have a common pool of dice, around fifty of them. They can use dice from this pool any time and add them to almost any skill throw. Since critical hits and failures don't exist in this game, I think it's a neat way to make some actions matter, by adding a bunch of dice in one time and create a really flashy and impressive feat. Basically, you're making critical any time you want to.
But there's a catch: every time you use a dice, the DM get one dice to use. And he can as easily make critical hits for his NPC's or negate your character's actions with that! Sometimes it's used to make some soldiers improve their Imperial Stormtrooper Marksmanship Academy way of being, but some uses are... much more dangerous. And it creates this weird situation where you don't want your character to be too succesfull, otherwise it'll come bite him in the ass later (no, the pool doesn't reset between games). I like this dynamic way of playing, and I've seen some DM create a "Lucky Point" to emulate this mechanic.
The best example I've got come from a game where we were a little heavy-handed in using those dice during a space fight. Our ship was slightly damaged, but the opponent's ship was downright broken into pieces, and we made so many impossible shots and stunts we thought we were invincible. And then, out of the blue, our DM puts back around 30 dice in our pool and says: "Your fight has create enough ruckus to attract a military-grade ship, and they want to arrest you. Roll initiative."
... After that incident, we started to use those dice more carefully.
I'm actually redoing this, inspired by one of the very few good ideas to come from my 4E gm. Roll 1d4 for tier, then 1d6 for exact result. Still only applies to combat. I hate the idea of insta-kill rolls, though, so I still won't be using them. Also note that starting next session, I will make it clear to my players that a sufficiently funny description of the action will lessen the penalty.
- 1-1: Reduced to -1 HP and dying.
- 1-2: Reduced to -1 HP and unconscious, but stable.
- 1-3: Reduced to 0 HP and disabled.
- 1-4: Character is blinded for the encounter.
- 1-5: Character is cowering for the encounter.
- 1-6: Character is sickened for the encounter.
- 2-1: Character takes 2 negative levels for the scene.
- 2-2: Character takes 1 negative level for the session.
- 2-3: KOM or KDM (whichever is higher) is cut in half for the day.
- 2-4: KOM or KDM (whichever is higher) is cut in half for the encounter.
- 2-5: KOM or KDM (whichever is higher) is reduced to 10 for the day.
- 2-6: KOM or KDM (whichever is higher) is reduced to 10 for the encounter.
- 3-1: Character is set on fire.
- 3-2: Character is bleeding.
- 3-3: Character is exhausted.
- 3-4: Character is fatigued.
- 3-5: Character is dazed.
- 3-6: Character is confused.
- 4-1: Critical hit on nearest creature within range that wasn't the target of the attack. If nobody is within range, apply damage to self.
- 4-2: Regular hit on nearest creature within range that wasn't the target of the attack. If nobody is within range, apply damage to self.
- 4-3: Base weapon damage on nearest creature within range that wasn't the target of the attack. If nobody is within range, apply damage to self.
- 4-4: No effect, if player describes their miss in a funny enough way.
- 4-5: Nearest enemy decides that the character is too amusingly incompetent to kill just yet and will not attack him for 1 round.
- 4-6: Nearest enemy dies of laughter.

Just an idea I had - any GMs out there who care to share their crit tables and the like? I think it might be fun to compare and take notes on some of the more amusing ones people have come up with.
I'll start off. The system I use, Legend, has a set rule for critical hits (add twice your level in damage), but doesn't have anything for failures. So I came up with the following table for combat critfails based on a d20, provided alongside examples of how I've flavored things when they've come up.
The only other amusing crit I've come up with happened when a character attacked with a whip and attempted to disarm a goblin. The attack hit, but the goblin critted the save for the disarm. So I had the goblin yank the whip out of the player's hands so hard that he hit himself in the face with the handle for 8 damage.