And if writing a 50,000 word novel in one month (National Novel Writing Month) [1] is bad enough, Anvil Press [2] holds a 3-day novel contest [3] every year (it's already been held this year sadly).
I don't think I'm up to that challenge.
But for the more visually talented people out there, you also have the 24-hour comic [4], in which you draw a 24 page comic book in 24 hours. I've read a few [5] and inevitably, the drawings get more and more loose and the story line (as much as there is one) gets less and less coherent. It might be interesting to draw one backwards—that is, start with the last page and work your way forward. That way, the drawings get tighter and the story gets more coherent as you read along.
Again, I don't think I'm up for that challenge either.
And then there's the 24-hour play [6] …
[2] http://www.anvilpress.com/
[3] http://www.anvilpress.com/3daynov/
[4] http://www.scottmccloud.com/inventions/24hr/dare/dare.html
[5] http://www.scottmccloud.com/inventions/24hr/index/index.html
[6] http://www.scottmccloud.com/inventions/24hr/plays/plays.html