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.
Tom: Dude! I can't figure this out! We are seriously fucked!
Mike: This is worse than the Water Temple!
"I've been getting Water Temple'd by work lately..."
Spouse B: Sure!
Then after an hour or two you figure out: "Holy shit! I can make the water rise!" You then think that your a fuckin' genius, but you're only 2% through the Temple.
When (if) you get to a room filled with water and a tiny little island in the middle, no, you are not high. Nor are you getting haunted by BEN. No, it's far worse. You're fighting Dark Link. After many failed attempts (even though you coulda used the Megaton Hammer), you get another Hookshot... But this time it's twice as long!
Now, it's all downhill from here. After you get the Boss Key and enter the boss dungeon, you're suddenly snuck up on by a tentacle monster that's gonna penitrate your ass! (Not)
After defeating Chaos- er... Morpha, you get you're next heart piece and the blue medalion!
2. Friend 1 :Hey remember the good ol days when we played Ocarina of Time
Friend 2 : Yeah! I do, hey remember how frustrating the Water Temple was?
Friend 1 : DUDE! don't even get me started on the Water Temple
2: Really? I skipped to the boss after I got the Longshot.