Consider an experiment that was done by Daniel Simons, a psychologist at the University of Illinois. He or a member of his lab would walk up to a pedestrian on the street and ask him or her for directions. While the pedestrian was responding, workmen carrying something large (like a doorframe) would walk between them. As the workmen passed, the experimenter switched places with someone else. Only half of the pedestrians noticed, even though the two people could be quite different.
Link. Test your own memory here. I scored a 2.6.