I have a tendency to hijack games as a player, sometimes. My GMs have been kind of passive for most of my gaming career, and I usually make rather headstrong characters that punch the story in the balls and grab the momentum. I think I'm getting better at not being an asshole, but if a GM tells my character it can't know something just because, I throw skill checks on him until he yields. Maybe not a healthy idea in the long run, but hey ho.
Like Wooberman, I've definitely fudged dice to not let the game be stomped all over. In the end, I just made min-maxed, ridiculously powerful characters to combat the PC's unbelievable roflstomping.
I once killed a player's character because he no-showed two times in a row. Yep, that's your character lying dead under the big ball of fuck you.
I don't think I back-seat GM very often, but if someone's about to lose a character or something else very bad for the player because of a rule snafu, I'll likely hop in.
I've never played DnD. Ever. Hoping to rectify this with the new edition coming in the summer.