The term false alarm originates from a mistaken call to the fire department reporting a fire, when there really is no fire. Also called a nuisance alarm, it could be a deceitful prank, or just worry over something that did not actually happen. A similar instance is the classic anecdote of someone yelling out, "Fire!" in a crowded theater, causing a dangerous stampede by the panicked crowd.
Another example would be the Aesop fable of "The Boy Who Cried Wolf", but that's another story.