Wow...some of these stories are amazing in many bad ways...
I've only walked away once. It was my first time playing any type of tabletop role-playing game. It was also the first and last time I've played in a 3.5 game.
Background Time: While I never played until this point, my girlfriend and I did hand out with a group of friends who did game every week. We were often there during the meetings, so we did know some of the rules. After about two years of us watching, the DM decides to let us join his new campaign he was having with different players. We agree and got to work making characters. I made a Lvl. 1 Raptoran Ranger while she made a Lvl. 1 Elvish Rogue.
The Event: The first thing that got us thinking that this was a bad idea was the first thing out of the DM's mouth. Even though we were supposed to start a new campaign, the other players were allowed to use their Lvl. 17 characters. We decided to run with it. We started the campaign in a bar. The first thing one of the player's decider to do (he shall be named "A") was go a w***house. The DM then spent the next HOUR explaining the events in great detail. At this point both me and my girlfriend are really creped out. But we still go along with it.
After that fiasco, we finally got our first fight...against an Elder Red Dragon. Since both me and my girlfriend were under-leveled for such an event, we were both made unconscious somehow (instead of outright dying) when the dragon attacked us. After the party defeated the dragon, "A" decides that he will now proceed r*** my girlfriend's character. The DM begins to describe while "A" begins to wink at my girlfriend. At that, my girlfriend slapped that a**** across the face and we left.
Thanks to this great introduction, it took me 6 months before I got back into Tabletop Role-Playing (my girlfriend and I had a falling out between said period. We're still friends). I have yet to reach a game were I just walked out ever since. But then again, I am still relatively new to this so...
(Sorry for the long post)