(First fourth edition PC loss!)
The PCs were exploring an ancient Eladrin ruin that had since become the home for orc and human pirates. Making their way through a few easy fights, they later found themselves in a running battle with numerous orcs - each orc running off to summon more warriors. Rather than running, the PCs decided to hold their ground.
Eventually, there was a standoff, with the Orc chief and one of his bodyguards on one end of the door, and the PCs on the other. When the PCs burst into the room, the enemies were blocking off the end of a long hallway.
So, they surge forward, and the orcs activate a whirling blade trap - essentially, a blade slices across a random segment of hallway each round.
Desperate to avoid the blades, the PCs, led by Tongs (a tough Eladrin Warlord) try to bullrush the orcs backwards, so they can escape the trapped halls. NO such luck, though the Minotaur Fighter was able to push the orcs back for a moment - but the orcs pushed back.
Rather than fighting, the PCs wasted all their efforts on pushing - ignorant of the fact that, for all the whirling death of the blades, no PC had been hit by one after four rounds of combat.
Sadly, both Tongs and his companion, Hammer (the aforementioned Minotaur) went down in the fight, and the rogue had to escape to safety.
The humour of it all comes from the fact that the dying Tongs, ignored by the orcs, was able to stabilize at negative hit points... and then happened to get hit by one of the whirling blades, killing him instantly.