Are you familiar with the platformer game, N?
(
http://www.harveycartel.org/metanet/index.html)
Many of the user levels beat themselves, and with N, it's many, many times more complex then those mario levels. N has a fairly advanced physics engine, so the springs/blocks/enemies/etc. must be tuned just right in order for the character to dodge rockets/bullets and reach the goal without any user input.
If you download the game, go to the user levels, and try out the levels of type "DDA" (It says "DDA" to the right of the name of the level, under the catagory "Type"). The "DDA" games are the type that beat themselves. Many of them are incredible, such as the level "Kablizzy's Cavern Number Three" or "Tremble". Some of the self-beating levels require the user to hold down the left arrow key down the entire time, such as the level "Leftorium". In addition, the game has a built in level editor, should you want to try your own hand at the art.
RPGillespie
[Edit] Leftorium is one of the most intricately tuned of the self-beating levels. It is uncanny how many lasers fly at you that you barely dodge, thanks to the author's careful placement. Ridiculous. Try that one out, if you get the chance.
Here's a video of one of the DDA userlevels. It's kind of small, and if you were watching it in game there would be much more detail, but just rest assured that there are a LOT of bullets flying at this guy.
http://video.google.com/videoplay?docid=3029814906769849110&q=N+game+DDA&total=4&start=0&num=10&so=0&type=search&plindex=2