The Broken Window Fallacy
This 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, benefiting the baker, who will then buy shoes, benefiting 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
The fallacy of the onlookers' argument is that they considered the positive benefits of purchasing a new window, but they ignored the hidden costs to the shopkeeper and others. He was forced to spend his money on a new window, and therefore could not have spent it on something else. Perhaps he was going to buy bread, benefiting the baker, who would then have bought shoes, etc., but instead he was forced to buy a window. Instead of a window and bread, he had only a window. Or perhaps he would have bought a new shirt, benefiting the tailor; in that case the glazier's gain was the tailor's loss, and again the shopkeeper has only a window instead of a window and a shirt. The child did not bring any net benefit to the town. Instead, he made the town poorer by the value of one window.