A while back there was some ABC news show about how unreliable eyewitnesses can be, particularly when they are involved with another activity.
Most likely Dateline NBC, Friday January 16, 2004.
They showed a video tape of three people tossing around different colored balls. They ask you to count how many times a certain colored ball was tossed. At the end of the test they didn't ask so much about the number of tosses, but about the gorilla. What gorilla? I thought it was a trick, but a replay showed that after a few seconds of ball tossing a guy in a gorilla suit came out and stayed on camera for several seconds.
The original video may be viewed at
Demo 15. Requires Java, and doesn't seem to work in Opera 8.0b2 with Sun J2RE 1.5, but does work in IE for me, with the same Java.
See also
Visual Cognition Lab. At the bottom there's a link to more demonstrations.