There's a very famous perception experiment, which many of you have probably seen. If you haven't, you should watch it here before reading on:
I will admit that the first time I saw it (at a conference), I totally missed the gorilla. Totally. Of course, once you've seen it or know about it, it's hard not to see it. And, by this point, so many people have seen it, that the overall video test has lost much of its power. So the folks behind it decided to see what would happen if they knew you were looking for the gorilla, and came up with the following:
Basically, they assumed that people were now looking for the gorilla, and got people to more or less focus on that, and miss the "new" changes. Once again, I have to admit I totally missed the changes. As the researchers are pointing out, this suggests that even if you're "expecting the unexpected," it's often difficult to notice it actually happening. As the researchers behind the videos note, many of the folks out there who use their original video to teach people to "expect the unexpected" are missing the point:
"A lot of people seem to take the message of our original gorilla study to be that people don't pay enough attention to what is happening around them, and that by paying more attention and 'expecting the unexpected,' we will be able to notice anything important," he added. "The new experiment shows that even when people know that they are doing a task in which an unexpected thing might happen, that doesn't suddenly help them notice other unexpected things."
The guys behind the videos have done some other unique experiments as well, which you can find on their website. The other one I really like is this experiment involving a guy asking a pedestrian for directions, where the guy asking for directions is secretly "switched" with someone else in the middle, and 50% of people don't even notice: