The first draft of the novel "Frankenstein" was written by Mary Shelley during a holiday in Switzerland, as a dare. Consequently my email group of writing friends (the Critter Litter) ocassionally have what we call a Shelley - a date to produce a story on a certain theme by a tight deadline. It doesn't have to be good, it just has to be done. The point is that the worst thing you write beats the best thing you never got around to writing, and you can always polish it later.
We held a Shelley recently, and the theme was slugs. It's not what I would have chosen, but I signed up anyway because it's high time I got back to work and I hope it would give me the required kick up the rear.
It worked. I wrote a very strange story which I finished a day late and I really enjoyed writing it. I think one reason was that I didn't think I'd have a hope of publishing it, so all the pressure was off. And reading it over, it needs polishing, but I think I can sell it after all.