So I build this trap, which I'm quite fond of. It's a five-foot-wide corridor, with a stone doorway at each end. When the party enters, thick adamantine doors fall down into the doorways, sealing them in, and the floor drops out. Since this was a high-level campaign, I had an antimagic field trigger when the trap springs, too.

The party comes upon a narrow corridor. The party druid is sad, because his bear cannot fit through the opening. So he casts reduce animal on his animal companion. He tucks his now teddy-sized grizzly bear beneath his arm and follows the rest of the party inside. The doors close. The floor drops out. The bear resumes its normal size.

The bear is now wedged in the corridor, forty some feet above the rest of the party, which has fallen into the dungeon complex below. A few goblins with class levels show up to hack at the PCs, and the PCs slaughter them. The PCs celebrate, and the bear pops out of the ceiling and lands on one of them.

...I guess you had to be there...