1. Make a wish (preferrably not serious)
2. Someone comes a long and comments to that wish, granting it, but making it horrible :D
"I wish I had a pixie living in my garden shed."
Unfortunately, it's a baby eating demon pixie. If you go anywhere near it you must offer the pixie newborn babies or it will eat you.
(stolen from corrupted_wish because i have no original thoughts :D)
Thank you lady_supernova for the idea!