The Shepherd's Boy and the Wolf

A Shepherd's Boy was tending his flock near a village, and thought it would be great fun to hoax the villagers by pretending that a Wolf was attacking the sheep: so he shouted out, "Wolf! wolf!" and when the people came running up he laughed at them for their pains. He did this more than once, and every time the villagers found they had been hoaxed, for there was no Wolf at all. At last a Wolf really did come, and the Boy cried, "Wolf! wolf!" as loud as he could: but the people were so used to hearing him call that they took no notice of his cries for help. And so the Wolf had it all his own way, and killed off sheep after sheep at his leisure.

You cannot believe a liar even when he tells the truth.