My worst gaming experience was a game of Star Wars. The game up to this point had been fairly unpleasant, as the GM insisted on rolling every. single. thing., and following the published modules precisely. What this meant is that we never, ever, succeeded a Climbing/Jumping roll, and thus generally ended up held together by Medpaks.
Anyhoo.
So we started the game about noon one weekend (ah, student life, when there was so much spare time...). Blah blah blah game game and so forth, until we get to the point where we learn that we have to help defend a planet from invading Imperials. With our starfighters.
Pity #1: only two of us have starfighter piloting skills
Pity #2: the GM decides to take the module's suggestion that you use the Star Warriors game to run the space combats. So instead of the D6 Star Wars quick'n'fun spaceship combat, we use the Star Warriors tedious wargame instead.
So the majority of the group sat around for uncounted hours while the other two actually fight their space battles. The fact that one of the battles had a "even if you win, you lose this bit" kicker in it was merely icing on the cake.
In the end, we all decided to man gun turrets for the final battle, even though one of us was playing a Kid (I'd put a child in charge of a a gun turret, really I would), and I was playing an [ahem] Ewok Jedi (look, if a little green dude with pointy ears can do it...) who was all peace-loving and stuff. But dammit, we wanted something to do!
The game finally ended at 8am. So 20 hours of gaming, about 15 hours of which was utter tedium.