Psychological experiment.

[SilentWolf]

izaaz101 said:
Assuming this is the video I saw ~5 years ago, it's an experiment on change blindness - how we are terrible at noticing big changes in our environment.

Yeah; I think it is meant to show that if we are told to concentrate on one thing, then we tend to miss many very obvious events.  In other words, if we are not told to look for it, then we seem to not notice it.