While reading Cracked.com the other day, I came across another great example of change-blindness. For those who don't know, this is a psychological phenomenon in which people fail to notice some change that has taken place around them. The basic explanation for this phenomenon is that we have a limited amount of attention. Thus, when we are paying attention to one thing, we often don't notice other things that are going on around us because all our attentional energy is already in use. Furthermore, when we aren't paying attention to something, our brain will fill in the details surrounding that thing we are focusing on. Thus, when a change occurs and we are focused on something else, we often don't notice it. This, for example, helps explain why texting while driving is such a bad idea. When we are focused on a text message we don't notice what is going on around us, and can miss changes in the driving environment (such as a car stopping in front of us) that demand attention.
In the following video we can see a great example of change blindness.
The basic explanation here seems to be that because the person in the video doesn't think he will see the person asking for directions ever again, the brain no longer pays attention to him, and basically "forgets" what he looks like. Thus, when a new person appears many people don't notice that a change has occurred. This is another fascinating exploration of human psychology and perception, and should once again lead us to a little epistemic humility about what we think we have seen and what we think we know.