The power of monsters.

[Shinizak]

It's no secret that a level 3-4 encounter could kill off even the most seasoned adventures if you just switch the abilities, feats, and strategy around...

Do you guys have any good combos to really screw with the players?

[Kol Korran]

i don't realy try to screw my party much, but long ago, back in 2nd edition, i had a strange combo: rust monster+ carrion crawler+ stirges. wasn't relay fair.

i prefer "shock value" over "kill value" though. you're the DM there is no challange in killing the party- you got all the tools, knowledge and the world behind you.

what is challenging is making a memorable encounter.

[Epinephrine]

I don't try to kill the party either. That's not really the point - you want to challenge them, frce them to adapt, come up with ideas, right?

But simply playing the enemies intelligently (when warranted) can be rewarding, and it's hard to complain about. My group has learned to hate groups of polearm wielders, and readied actions.

[RTGoodman]

Dungeonscape, co-authored by our very own Giant, has a whole section on this kind of thing. I can't remember any off the top of my head specifically, but I dimly recall by pretty impressed by the "new and improved" Owlbear.

[Devils_Advocate]

If you really want to screw with your players, you don't want to leave them thinking "There's no way we could have beaten that." You want them to think in retrospect "Oh, man, we totally could have won that encounter if we'd been smarter, but we were idiots."

"A good GM is bound by his own desire for fair play. [T]o kill a player or two in an encounter that is close enough that the remaining adventurers can pull through to win or escape... that is his holy grail."