Talk about your critical fumbles...

[Detoxifier]

So my 13th level character was nearly choked to death by an angry merchant because I rolled six...SIX...natural 1's in a row. What should have been a minor inconvenience from a low level non-combat NPC quickly turned into a life threatening situation, made all the more hilarious by the fact that my PC is a half dragon (3.5) and is being choked by someone 3 foot shorter than him who should ostensibly be of no threat.

Talk about your odds. My DM tells me I shouldn't buy any lottery tickets anytime soon.

[WilyQuixote]

Years ago, using the Champions system, in the world of Greyhawk. My PC is a Fighter and I happened to be the party spokesman in this encounter with a collection of bugbears. I'm trying to negotiate a peaceful resolution to let us pass through their territory on our way to our true objective. Well negotiations were not going well thanks to some flubbed rolls on my part. On of the PCs in the group had somewhat of a rivalry with my own and in character, in front of the bugbear chieftan, was giving my hero no end of grief for failing miserably at this task.

This was in my younger days so my mindset at the time was that of many young gamers. If at first you don't succeed try combat to resolve the problem. I roll to fast draw my sword and attack the chieftan. Predictably I roll the lowest possible result and fumble my sword in front of the chieftan. The sword goes flying off in a random direction. The DM rolls the scatter die and a to hit roll, and as you might be guessing by now, my sword hits the PC who was antagonizing me during the whole negotiation. DM rolls damage and gets one less then max damage! Nearly kills the PC, she was in the negatives and needed serious medical attention or start making up a new PC territory.

With out missing a beat I just look at the bugbear chieftan with a look of "Yeah that just happened." DM likes it, has me roll a presence attack (Intimidation skill roll) and then I get nearly max effect on the dice. The chieftan claps my PC on the back with a hearty laugh, steps aside, and allows us to pass through his territory unmolested.

Everyone else at the table looked at the DM and asked "What gives?". The DM says from the chieftan's perspective the swordsman just cut down one of his own men for insolence. That was something a bugbear chieftan could admire and respect!

[revaar]

My group from high school plays on Roll20 since we live in different states now. The die roller on Roll20 hates us. Even worse, our GM uses the critical fumble deck. If a combat passes without someone stabbing a party mate in the back, or someone throwing their weapon halfway across the battlefield, it is a minor miracle.

The worst one we recently had is when our sorcerer tried to disintegrate something. Getting a 1 on his roll to beat SR, the GM ruled that a crit fail with a spell. He draws a card from the magic fumble deck. The spell affects everyone other than the caster in a 60' radius. We all take full disintegrate damage, and would have likely died had it not been for our life oracle pumping around 70 HP into everyone on his turn.

Spoilers for Shattered Star:

Another time, my Archaeologist Bard, who is using a whip with minor sentience taken from a powerful undead creature(aka the Black Rider's Spine Flail, which the GM ruled i could use as a whip) crit failed an attack against a clockwork golem. GM had me make a UMD roll, which I also crit failed. For the next few minutes, The whip would only deal Non-lethal damage. Not very useful against golems. I ended up having to try and trip it for the rest of the fight.