The worst I can remember was the first convention I attended. I had signed up for a day-long game - a mistake - and the GM did not have pregenerated characters. When it was time to break for lunch, some people were still working on their characters. Needless to say, I decided not to return for the afternoon session.
Another low point was an Ars Magica game I played in. It would probably have been best if I had had the sense to not play, because the GM and I were not a good fit. I'm the kind of player who likes to make things up hoping that the GM will run with it, but the GM liked everything scripted to the letter and relentlessly railroaded us to the point that I felt that I couldn't affect the outcome of events in any meaningful way. It all came to a head when the GM felt that my character needed to be gotten rid of for the good of his campaign. Our characters were granted an audience with the king (I can't remember why) and in his presence when the following exchange occurred (I remember most of it word for word):
Me: A thought balloon appears above my head, "The king is a goblin-sympathizer!"
GM: You say that?!?
Me: No. I said "a thought balloon appears." I was thinking it.
GM: Well if you thought it, you probably said it.
Me: No. I thought it.
GM: Well, you probably muttered it under your breath.
Me: Who's playing my character here, you or me? I said I thought it.
GM: Okay... You notice an amulet around his neck. He's read your mind!
End of character. And end of my involvement in the campaign. It still makes me laugh, though.