The Mimic.
A few years back our GM had us running through a forest when a massive rainstorm hit. We took refuge in an old cabin we had spotted hours earlier, but hadn't checked out. We fixed the hole in the roof and bedded down for the night. Unfortunately the bed, chair, back door, an old foot locker and an end table were all mimics.
We had APL of 8 and had a pretty powerful wizard. We lost our rogue, my Barbarian lost his +2 Keen great sword and we barely made it out alive.
Ended up having the wizard spam the cabin with fire balls when the rain stopped.