It's worth noting that if you haven't played Diablo 2, you should probably track it down and play it. It's pretty damn entertaining.
Toward the end of the second act of Diablo 2, you get the news that the Prime Evil Baal is chilling out underneath the desert city's palace. After a bit, you come to a hole in the wall which, once opened, promises to lead you to what, at that point, seems like a final boss battle.
Only it obviously doesn't. It's way too early in the game. The setup is so obvious it's almost painful, but something's beyond that hole in the wall, so you enter, anyway.
|Oh. Uh... Hi........ I'll see myself out.|
A demonic voice growls "Looking for Baal?" and suddenly you're embroiled in one of the more difficult boss fights in the game. Duriel is ridiculously fast and his hits whittle your health to nothing in moments.
It seems cheap, but the game actually uses your genre-savviness against you. The first boss was a relative breeze, and nothing to this point in the game had been too much of a challenge. You know that - whatever ends up being in that room - it's not the final boss, so you relax a bit. If you weren't prepared for a boss like Duriel (and you almost certainly weren't), your first encounter lasted about 3 or 4 seconds.