A story from Zen literature told in Buddhism Is Not What You Think by Steve Hagen, p30:

…a fellow comes upon a demon for sale at a carnival – and very cheap, too. “What can it do?” he asks. “This demon will do anything for you,” the merchant tells him. “Just tell it what you want done, and it will do it. It will do your laundry, cook your meals, do your shopping. It will handle all your chores around the house. The only thing to remember is that you must keep it busy.”

The man, focused on his immediate concerns, thinks this is a bargain. He buys it and takes it home.

At first, all is well. Just as promised, the demon quickly goes to work, taking care of whatever the man commands it to do. The demon fixes the roof, cooks meals, and plants the garden. Of course, the man has to keep coming up with things for the demon to do. Yet this seems doable – until, one day, the man has to step away briefly on personal business. When he returns only a short while later, the demon is roasting the neighbor’s child on a spit.