One of the first things I learned in a college journalism course was just how unobservant we are, even when we think we are paying attention. This lesson involved having someone run into the classroom, grab the instructor's briefcase, and run out of the room. Versions of this exercise abound. In my class, no student correctly described the intruder. There were various descriptions of his clothes, height, physical appearance, facial hair, etc. How so many people could honestly describe the same person in such varied way has always amazed me.
With today's heightened security, this type of exercise is probably not done anymore.