One of the mapping forums I frequent recently held a competition to design a puzzle requiring the player to get past a locked door. You could use any engine and had to stick to the template provided (with additions/minor alterations allowed as necessary).
Original thread with rules and an example: http://forums.mapcore.net/viewtopic.php?f=57&t=16225
I decided to try something out of the box in Gears of War, while trying to add a mix of humor into the final map. Of course it had to be absurd and over-the-top in true Gears of War fashion. Here's what I came up with after putting about 12 hours into it.
Bear in mind this was created with the intent to be short (about 2-3 minutes long).
Download (4.7 MB): http://dl.dropbox.com/u/4783596/MapC...orSerenius.zip
Youtube Video with Solution: http://www.youtube.com/watch?v=pRP8XAY5bqY