Sometimes a good DM gets tired of throwing another handful of monsters at a party over and over again and decides to get a bit crafty, a bit devious, perhaps even a bit naughty, and will set up a cunning, vicious trap for the adventurers to discover, disarm and bypass safely - or not.
There are a number of useful resources
available, but what are some of your
favorite traps and/or puzzles you have created or encountered? What do you do when you create them? What makes for a good puzzle, and what makes for a bad one?