CRY WOLF

To cry wolf is to complain or ask for help when you don't really need it. 

It comes from the children's story “The Boy Who Cried Wolf,” where a shepherd boy called for help - cried "Wolf!"  when there was no wolf attacking them. Then later when help was needed because the sheep were being attacked, he got no help because no one believed him.

image courtesy of dreamstime.com

WHAT'S THE CATCH

What's the catch? This means what is the drawback, the disadvantage, or the hidden problem. Someone wonders this if something seems or sounds too good to be true.