The Parable of the Broken Window
Wikipedia has a good article on the broken window fallacy.
The parable describes a shopkeeper whose window is broken by a little boy. Everyone sympathizes with the man whose window was broken, but pretty soon they start to suggest that the broken window makes work for the glazier, who will then buy bread, benefitting the baker, who will then buy shoes, benefitting the cobbler, etc. Finally, the onlookers conclude that the little boy was not guilty of vandalism; instead he was a public benefactor, creating economic benefits for everyone in town.
