The Boy Who Cried Wolf problem?
In the fairy tale, aShepard boy is bored by watching his sheep. To entertain himself, he cries out that their is a wolf approaching. The townspeople run out to drive off the wolf, but find that their isn't one and that the boy was lying. This happens several more times, until a REAL wolf appears. The boy cries out that their is a wolf, but the townspeople don't believe him, thinking him to be lying again. Depending on the version, the wolf either eats all the sheep, or eats the boy.
The moral of the story is that someone who lies a lot becomes more and more difficult to believe, and in the end this could mean that when they are really in trouble, no one will help.