"Change Blindness"

In visual perception, change blindness is the phenomenon that occurs when a person viewing a visual scene apparently fails to detect large changes in the scene. For change blindness to occur, the change in the scene typically has to coincide with some visual disruption such as a saccade (eye movement) or a brief obscuration of the observed scene or image. When looking at still images, a viewer can experience change blindness if part of the image changes.

There's a great example of this on youtube, but they had disabled the embedded video option, so click here to check it out.

The study of how the brain actually works has always amazed me. Out of all God's creation, that's probably the most astounding to me.