Old research, still funny. Dr Daniel Simons of the University of Illinois and Dr Daniel Levin of Vanderbilt University created nice setups to trick humans, based on attention.

The setup they created is: tell somebody to watch this video of basketball players and to count, how often the white team has passed the ball.

the video:
http://viscog.beckman.uiuc.edu/grafs/demos/15.html

At the end of the video,they ask: how often? And the (but don't spoil this by telling it before): Did you notice the gorilla walking from right to left through the scene, banging on his chest?

An article on that.