While I am not as articulate as other posters and my memory is that of a 90 year old man, I will attempt to recall events of the worst session I have ever had the displeasure of participating in.
-My story takes place in the summer of '98. Our regular group consisted of 6 or 7 close friends who had been playing together, under the same Gm (also a good friend), for about 3 years. We ran the gamut of player types, which was sometimes a bad thing, but we generally had a very good time. Anyway.. one of our players- let's call him F- was your classic hyper macho persona: crude, racist, sexist, and violent, and he would come to the be source of our woe.
We were playing Ravenloft (with evil characters allowed), attempting to escort a gypsy prince and princess to sanctuary.
Our reward would be escape from the mists, or so our patrons claimed. We had almost reached our destination, when we were attacked by thugs sent by the Lord of the Land. Battle was going well, but due to bad rolling we were not as quick in dispatching them as we should have been. The LoL showed up and, realizing that he could not harm the children without their protectors willingly giving them up (some of us were evil, yes, but we wanted out more than anything), he teleported himself and the entire party back to his keep to "negotiate". My character, a neutral evil necromancer, was the first to be given an audience.
Well, I played the gypsy card without hesitation, agreeing to give their life forces to him willingly, on the condition that he let myself and my comrades leave his realm unharmed. Unfortunately, it wasn't as easy as that. He demanded one additional sacrifice.
All of our prior role play was done out in the open but, at the mention of an additional sacrifice, I pulled out the old note pad. Everyone got very quiet as I slipped the paper to our gm with my reply on it. It read something like this:
I tell the Lord: "the wretched truths of necromancy have left my soul in tatters. I would be of no use to you, my lord, but the healer, he is ripe with spiritual potency. I give him to you freely" I sacrifice F's character.
At reading this, our Gm explodes with laughter. Another player, sitting opposite me, tears the note from his hand to see what was so funny. He begins laughing hysterically as well. The other players, looking puzzled, are informed that F's character vanishes (they are in a separate room awaiting audience). With that, the real-life mood got noticeably more tense, as F unsheathed a large knife, and was grinding it into the table, looking at me as if he were going to leap any second. The gm began to physically threaten him, the other players were attempting to talk him down, and I simply excused myself to get a coke. With me gone from the room, the situation was, thankfully, diffused. F left, I refused to ever game with him again, and that was that.
I regret having put F in that position, I was just too short-sighted to consider his feelings. High-school gaming was immaturity and naivete at it's best, but this single event helped to sober us up real quick.