This isn't a "bad decision" story, but I felt the desire to share anyway. This story explains why I hate fumble rules.
The first time I ever played with fumble rules was my second time playing D&D 3.5, 7 1/2 years ago. I was playing a cleric in "the world's largest dungeon," with two other players. I don't really remember a lot of it, but I do remember at one point we were fighting a displacer beast.
See, I had just healed one of my teammates, and without anything else to do, decided to wade into melee and whack the creature in the head with my mace. Unfortunately, the dice gods were not with me that day, because I rolled a 1. Playing with fumble rules, my GM said "roll again to see how badly you missed." So, I toss the die on the table, and after it bounces haphazardly on the smooth surface, it spins and finally settles... another 1.
At this point, I know something bad is going to happen, and I wait for a ruling from the GM. He takes a few seconds before telling me, "Your swing goes wide, hitting your teammate standing next to you. Roll one more time to see how badly you hit him." Still new to the game, I hadn't as of yet developed any dice rituals, and maybe that's what sealed my fate that day. After juggling the D20 between my hands a couple times, I tossed it on the table, where it bounced around, nearly rolling of the table. Finally, stopping precariously on the edge, the result on the die made my heart sink. I'd rolled a 20.
The GM chuckled at my misfortune. "Well, it seems what happened is that as you swung your mace as hard as you could, the head you were targeting disappeared. The force of your swing hitting nothing but air threw you off balance, causing your weapon to continue its momentum right into the skull of your adjacent ally. Roll for damage." And wouldn't you know it, that little D8 decided to roll for maximum damage. All the players at the table let out a simultaneous groan as we calculated the damage against the character's remaining hitpoints. The hitpoints I'd just restored back to him last round.
Congratulations, Jason, you've just killed your own teammate. Here's your Worst.Healer.Ever award!!