Change blindness refers to the failure of an observer to notice large changes in a scene. For change blindness to occur, a brief visual disruption or a disruption of the scene is typically required.
Via the Situationist Staff, the video below depicts a change blindness experiment by Daniel Simons and Chris Chabris. If you aren't familiar with this research, you should definitely check out this clip.
One more from Daniel Simons: Take a 30-second visual attention test.