Considered by many to be the equivalent to a complete rectal examination.
The Water Temple from Zelda 64, which has to be the hardest, most difficult level known to man. Ever. Filled with traps, mazes, puzzles, illusions, and who knows how many times you have to raise and lower the water level.
Every gamer I have spoken with has only rude, vulgar things to say about the Water Temple - and many stories of trial and error - using the wrong keys on the wrong doors, saving and screwing up in the wrong places, forgetting which rooms were which... it's a hellhole.
It is also used outside of the gaming world to describe a problem that is virtually impossible to solve.
The only thing I don't get is why they put the hardest level in the middle of the game.
I spent four fucking hours in the Water Temple and I still have no idea where the fuck I'm going.
Tom: Dude! I can't figure this out! We are seriously fucked!
Mike: This is worse than the Water Temple!