The historian of Science Thomas S. Kuhn describes a disturbing experiment conducted by a pair of psychologists in the 1940s. Subjects were given glimpses of playing cards, one at a time, and asked to name them. There was a trick, of course. A few of the cards were freakish: for example, a red six of spades or a black queen of diamonds.
At high speed the subjects sailed smoothly along. Nothing could have been simpler. They didn’t see the anomalies at all. Shown a red six of spades, they would sing out either “six of hearts” or “six of spades.” But when the cards were displayed for longer intervals, the subjects started to hesitate. They became aware of a problem but were not sure quite what it was. A subject might say that he had seen something odd, like a red border around a black heart.