Change Blindness -- Illustrated by Derren Brown

UX Magazine posted an article about "change blindness" a while back, illustrated by a video of a Harvard study and by Derren Brown's classic on-street illustration of the effect.

The idea of "change blindness" (see Wikipedia for more) is that when we are made to focus on particular details, we miss other obvious pieces of information that are presented to us. In the videos posted by UX, observers fail to notice that the person they are speaking with has turned into another person, because they are so focused on another fact/task.

While change blindness is obviously useful to an illusionist, being aware of the effect can be worthwhile for anyone.