The Boy Who Cried Wolf
The Boy Who Cried Wolf is number 210 in the Perry Index. The fable is about a shepherd boy who keeps tricking people into believing that a wolf is attacking his flock of sheep. The boy tricks the people so many times that when a wolf actually comes to kill the flock, the townspeople don't believe his cries for help and the wolf eats his flock. The moral of the story is that this is how liars are rewarded: even if they tell the truth, no one believes them. The story dates back  to Classical times.