From our friends at Futility Closet:
In a 2005 experiment, psychologist Petter Johansson and his colleagues presented each subject with two photographs of women's faces and asked which they found more attractive. In each case the experimenter then presented the "chosen" photograph and asked the subject to explain their choice. But in fact, using sleight of hand, the experimenter had exchanged the photos and was presenting the one that the subject hadn't picked.
Only 13 percent of the subjects noticed the change; the rest went on to confabulate an explanation justifying a choice they hadn't made. And yet, in post-test interviews, 84 percent of the subjects said that they would have detected such a switch if one had been made.
A subsequent experiment involving supermarket taste tests of jam showed the same effect: Subjects indicated an initial preference and then (after the samples had been surreptitiously swapped) failed to recognize that they were now tasting the rejected variety and went on to justify that choice.
Johansson and his colleagues called this choice blindness. "We do not doubt that humans can form very specific and detailed prior intentions," they wrote, "but as the phenomenon of choice blindness demonstrates, this is not something that should be taken for granted in everyday decision tasks."
(Petter Johansson et al., "Failure to Detect Mismatches Between Intention and Outcome in a Simple Decision Task," Science 310:5745 [Oct. 7, 2005], 116-119.) (Thanks, Colin.)