This is something that happened to a friend of mine. Let's just say the GM had a bit of fun over a player's expense (but it was totally justified considering)
So, my friend used to play an Elf named Denelian, and this Elf made horribly bad life choices. You could say that this particular story started with him wading through a swamp for some reason, and a giant centipede took a chunk out of his leg. He needed lots of stitches and medication, but overall, he recovered fairly well. His doctor did tell him he needed to take a certain medication regularly.
He forgot to take this medication regularly.
Fast forward a few weeks. He and his fellow adventurers had hired mercenaries to storm a garrison, to kidnap a certain someone. But first they needed to cross a perilous swamp by horse. They all agreed to make Denelian the leader for this expedition for some reason.
The moment he set foot inside the dark swampland, the GM asked him to do certain rolls. He continuously failed these, and had to mark the failures in his character sheet.
So, riding in front of 20 mercenaries and 4 friends on his trusty horse Illiana 3 (a story in itself, let's just say Illiana 1 and 2 are not with us anymore), in the middle of the swamp, Denelian finally fumbled yet another hidden roll. A giant caterpillar crawled up on his horse, and in a panic, he started cutting away at it.
Unfortunately, he was just crazy bananas and stabbing his own horse, which created a massive panic among the other horses, throwing their riders off and storming around, disturbing any and all horrors lurking in the murky swamp waters, killing nearly everyone in the party.
His madness killed like 20 people. Because he didn't take his meds. Who'd have known getting your legs nearly ripped off by caterpillars in a swamp would cause a phobia for swamps?