Wishbone: Too many to count. It's a rare occurrence for me to complete a game.
One that sticks out in my mind is Psychonauts. I never could get through The Meat Circus. I've been thinking about installing the HIB version, as apparently they've tweaked it a bit here and there, among other things making The Meat Circus slightly less impossible.
I am actually replaying psychonauts just for the meat circus. I bought it when it first came out and had some problems in earlier levels but for some reason the meat circus i found rather easy and cleared it in one go. Then you read all these stories about how difficult and frustrating it was and i watch dsp on youtube and he struggled there too (but he struggles a lot). So i thought i have to give this awesome game another run and see if i was just really lucky in my first playthrough. I am not there yet but if it is tweaked now it isn't really representative any more.