Subliminal marketing hoax
Interesting story debunked by snopes: http://www.snopes.com/business/hidden/popcorn.asp
In 1957, marketing researcher James Vicary said he had included subliminal messages such as eat popcorn and drink Coca-Cola in public screenings of Picnic for six weeks, claiming sales of Coca-Cola and popcorn increased 18.1% and 57.8% respectively. However, Vicary later admitted there had never been any such messages and his announcement was itself a marketing trick.