I've found that some of the most devious and player infuriating things you can do don't involve direct damage. Play on what they vocalize as their perceptions and split them up or at least foster them not cooperating 100%.
As an aside Sjeng you may like this brief story. For a D&D dungeon I have set up a 10ft wide corridor that has a pit trap in it. The pit is 10ft wide making it a 2x2 square section. Instead of the pit trap actually being where they see it it is behind. The visible trap is only an illusion and its actually solid ground

. So when the characters (at least those I play with) 'naturally' decide to jump it to cross they are jumping over solid ground. But the best part is the trap is the 10 square feet behind it. If they miss and land too soon the land on ground and no harm done. They get laughed at and generally go on without thinking for more traps. If they succeed they jump over solid ground and fall through the floor into the pit. Anyone that's watching the illusion can see the pit and their friend land smack dab in it. That's when I yell surprise

! Adjust with nastiness at bottom of the trap IE Spikes, Oozes, or even the fearful Water. Everyone always forgets about water and how unfriendly it is to someone in 100lbs of 'protective' plate mail, shield, and locked gauntlets... But that's another story.
| Normal Floor | Illusion Pit / Real Floor | Illussion Pit / Real Floor | Real Pit / Illusion Floor | Real Pit / Illusion Floor | Normal Floor |
| Normal Floor | Illussion Pit / Real Floor | Illussion Pit / Real Floor | Real Pit / Illusion Floor | Real Pit / Illusion Floor | Normal Floor |
Sorry about the Non-HQ interruption.
Edit: drathe did you just add that table function, or am I just blind?? Took me a bit to figure out but can be quite handy.