My own mistakes:
My second campaign I had the players try to find an evil magical sword of great power so that they could destroy it. As they are climbing out of the underground city, the player who has the sword decides to use it to kill the rest of the party. Lesson #1 in magical game balance.
Then of course there were the vampire monks I threw at my party before learning of the errata to the vampires energy drain. Had to tone them down a bit so they didn't slaughter the whole party. Of course, I really like part of that adventure. They're walking through an underground complex, and they keep finding holes drilled in the walls. What are these holes for? they keep wondering as they carefully map their location. After killing some of the vampires' minions, a couple of the vampires attack. They get smacked around a little bit, and then turn gaseous and retreat through the holes in the wall.
Then there was this Rolemaster GM I had:
Rappelling down an 800 foot cliff took six hours. Climbing a 1200 ft. mesa took four days.
My original character for that campaign was the son of a prostitute. After initially allowing this, he vetoed it on the grounds that his campaign was based on early medieval times, so there was no money. Since there was no money, there were no prostitutes.
One day he wanted to play INWO instead of Rolemaster, so he had us make riding rolls and hiding rolls for every hour of travel over the course of the week, along with random encounters like a bag full of dead ferrets. He did this until we got so sick of making stupid pointless rolls that we caved and played INWO instead.
There was only one city in the whole world in his campaign. Why, you ask? Because only the dwarves knew how to build city walls, and they wouldn't let anyone else do it. ???