Take this test of your awareness skills, then come back for discussion. Spoilers/hints are below so take the test first before you read any further!
A couple of people already mentioned this test in comments elsewhere, and I posted it to CommuteByBike yesterday. I passed the test and saw that "surprise ending", but I knew what to look for because I was familiar with the UIUC perception study from a few years ago that Sue mentioned.
Yeah, Fritz. Me too. Both on passing the test and on being familiar with what was going to happen. For more videos that test perception try this site. Select "The Videos". I like the "Colour-changing card trick", but they are all good. (wrong link on first posting)