It's from Breakthrough Advertising by Eugene Schwartz.
"...This is the reason that you cannot always use the most powerful claim in your headline. Or even the very problem that your product solves. Because without supporting evidence already existing in the mind of your porspect to prepare him for the headline claim, he just won't believe it. Either he'll believe that it's exaggerated or false, or he just won't believe it applies to him. In either case, too many of your prospects turn away without reading on, and your ad fails.
This fact - that your most powerful claim does not always make your most powerful headline - is a paradox that many cop writers still cannot accept."
This was in the section on "belief" as in, you cannot violate your reader's belief system or he will be gone faster than you can say "buy!"
I am still digesting this one as I've heard well... quite the contrary. What are your thoughts?