http://www.youtube.com/watch?v=bd8ppk0UCx8&feature=player_embedded
I love this commercial. They make a change to the time of the flash mob, and the guy doesn't know... so he tries to start it early. So make sure you make all changes ahead of time, and have and back up situations planned as well. And another hidden thing in the video is how obvious it is that they're all there together with matching trench coats. Don't make the costume obvious until everyone is together and don't match before hand either!