Perpetual motion machine
Charlatans and frauds purporting to have invented a perpetual motion machine — a machine that violates the laws of thermodynamics by generating enough energy to run forever on its own movements — have popped up throughout history. Perhaps none of these machines are as famous as Charles Redheffer's.
Redheffer's machine was so convincing that skeptics agreed to pay a hefty fee to prove him wrong. Sure enough, after removing a few wooden planks from the device, they found a belt which went through a wall where an old man was hiding — turning a crank with one hand and eating a loaf of bread with the other.