A shepherd boy, bored watching sheep, cried "Wolf!" to see the villagers come running. They found no wolf, and he laughed. He did it again, and again they came for nothing. When a real wolf finally appeared and the boy screamed for help, the villagers ignored him, sure it was another trick. The wolf scattered the whole flock.

Moral: A liar is not believed even when he tells the truth.

From Aesop's Fables, public domain.