Oh geez. I have like, a lot. But I'll list my top three.
In high school I let a friend who was a notorious powergamer to play a spellfire wielder in Forgotten Realms. If you know what that is, yeah, you understand.
I wimped out of killing a character, and everyone knew it. Evil priestess cast a spell, I rolled the dice, the PC and the DMPC were going to eat death. Then I said "You know what, she does something else". I think I made myself look really weak.
Same campaign, later. The PCs were dealing with an elven druid who had went insane. It came to light that she had went insane becasue she had an accident, miscarried, and just "snapped". The female player in the game got very upset with me.