Mimic Chests. They seem to be in most every freaking RPG in one form or another, usually sitting off alone in the corner of a dungeon, just waiting for some poor group of fools to come along...
I'm sure we've all had this experience at one time or another: Your party's hacking through the dungeon with some difficulty, leaving a virtual pile of monster corpses behind as they decend deeper and deeper...suddenly, "Hark! A treasure chest! Well, I'll just open it and...", and the chest is a Mimic!
Suddenly, you're fighting for your very lives, because the evil programmers decided to make it a good five or six levels* stronger than all the other monsters, this in addition to the fact that you can't run.
The particularily fiendish ones (programmers) also have the chest have a poisoning attack as well, making you fear all chests from that point on.
If you happen to play Seiken Densetsu 3
(great game), the chests have a little roulette system going on, and a Mimic icon appears occasionally. If you get it, watch out.
Those buggers are the same level as the strongest person in your party. Later in the game, you have the pleasure of meeting a Kaiser Mimic, and if you get that, you might as well reset the game.
*Results may vary.