You've probably heard of the phenomenon called change blindness. If conditions are right, large changes can occur in the visual field, and people will not notice them. Here is a demonstration of one of the classic change blindness scenes, used in experiments by Ronald Rensink. If you haven't seen it before, stop reading here, and try it.
If you didn't notice, the image blinks, and afterwards, a big piece of the airplane is missing -- the engine. Most people miss the change in this and several other experimental situations, and what's more, when shown the change, people are likely to say that they would not miss it1, demonstrating that we're not aware of our own change blindness. All of the laboratory demonstrations of change blindness are cool and all, but the coolest change blindness experiment involved a real world situation in which none of us would have predicted change blindness. The experiment was conducted by one of the researchers who won this years Ignoble in psychology, Daniel Simon2. Here's what happened: an experimenter approached people on the sidewalk and asked for directions. The experimenter stood in front of the person giving directions, and as that person was giving directions, two workers (who were actually experimenters) walked in between the experimenter and direction-giver holding a door. As the workers passed with the door between the experimenter and direction-giver, the experimenter who was receiving directions was replaced by another experimenter. About as often as not, the person giving directions continued to do so, and didn't notice that the person to whom he or she was giving directions had changed!
I know what you're thinking; this could not have happened. If the person to whom you were giving directions was suddenly replaced with a new person, you would notice. Well, at least some of you are wrong, victims of the same meta-cognitive error that causes people to think they would notice the engine appearing and disappearing in the Rensink demonstration. If you still don't believe me, check out the video. You can find it here. With experiments like this, Simon may have the Ignoble wrapped up for life.
Note: The runner up for Coolest Experiment Ever is Ehrsson's "rubber hand experiment." Any suggestions for second runner up?
1 See this paper.
2 Simons, D. J., & Levin, D. T. (1998). Failure to detect changes to people during a real-world interaction. Psychonomic Bulletin and Review, 5, 644-649.