Fool me once, shame on you...

fool me twice, shame on me. From WSJ.com: Paul Ehrlich has never been right. Why does anyone still listen to him? Excerpt:

[Ehrlich] is a reverse Cassandra. In "The Illiad," the prophetess Cassandra makes true predictions and no one believes her; Mr. Ehrlich makes false predictions and they are widely believed. The gloomier he is and the faultier he proves to be as a prophet, the more honored he becomes, even in his own country.