"I would advise you to buy it, because if you read it too long without handing over money you will find yourself the object of a Thief's Curse..."
-- Albus Dumbledore (QA)
An unspecified bit of nastiness which can be cast on someone who steals something.
References from the canon
- People who stand too long reading Quidditch Through the Ages in a shop without buying it might find themselves the object of this curse (QA).