"Shortly before the Patrician came to power there was a terrible plague of rats. The city council countered it by offering twenty pence for every rat tail. This did, for a week or two, reduce the number of rats—and then people were suddenly queueing up with tails, the city treasury was being drained, and no one seemed to be doing much work. And there still seemed to be a lot of rats around. Lord Vetinari had listened carefully while the problem was explained, and had solved the thing with one memorable phrase which said a lot about him, about the folly of bounty offers, and about the natural instinct of Ankh-Morporkians in any situation involving money: “Tax the rat farms.”

-from "Soul Music" by Terry Pratchett

A sort of Cobra Effect of the Fantasy World?