Subliminal (read this!) advertising
I remember hearing about the famous popcorn subliminal messaging when I was a kid. But I never knew that it was an admitted hoax.
The great site Snopes says: As usual, the media (and thereby the public) paid attention only to the sensational original story, and the scant coverage given to Vicary's later confession was ignored or quickly forgotten.
So the country got panicky and worried about something that doesn't even work. CNN wrote a nice summary of the subliminal issue in 2000.
So at least we don't have to worry about that anymore. And we can move on to more pressing concerns -- like all those backwards messages in rock songs!