In my campaigns I have two rules, don't annoy the DM and don't do stupid things or your character will die. Unfortunately, my players tend to ignore the latter of the two and tend to die early on in the campaign and then get much more careful and clever.

One of my favorite ways I've killed a party is with "rocks fall, you all die." While usually "rocks fall, you all die" is reserved for the breaking of rule #1, this time it fell under rule #2. You see, the party was hunting an evil lich in his old crumbling castle to destroy him before he completes an ancient ritual to summon a powerful pit fiend (I know a bit cliche). When they found him and his lower level vampire minions performing a ritual in the deepest dungeons of the castle they decided to disrupt the ritual, but there was a magical barrier protecting the lich and the vampires from magical attacks. So, the cleric had a brilliant idea, use earthquake next to the ritual because I'd ruled for a long time that antimagic fields and the like don't stop physical effects caused by magic and a tremor is defintely a physical effect. So, they cast it and, did I mention it was an old crumbling castle?, rocks fell and killed everyone, including the lich since his phylactery was part of the ritual.

Another one is one of my favorite traps. It's a pit trap full of widely spaced spikes with shiny things at the bottom. When the party decides to investigate using feather fall or fly to get down there safely, a gelatinous cube falls on them. This automatically engulfs them and usually ends up killing some of the party (paralyzed people can't fly) if they have fly on or is usually total party wipe if they don't.