The classic subliminal advertising apocrypha is this:
A cinema shows a movie trailer before the feature starts. It features a desert landscape from the cowboy Western movie it's advertising. The cinema owner deliberately slips single frames of a can of Coca-Cola, covered in carefully lit condensation. The idea is that the audience don't register the glistening can of Coke consciously, it appears for only a fraction of a second. But subconsciously, they know the Coca-Cola logo so well that the image does gently suggest, in combination with the desert scene, that the audience members might do well to go out into the lobby to buy something that will quench their thirst.
