One of the things that have made Jimmy Fallon famous is the classroom instrument performance of his musical guests biggest hit. He has done this multiple times, but something happened during this performance I have never seen before.
Look at the 1:08 mark where Brian Latrell takes the "knock block" I guess, and shows Jimmy that he was not really playing it right. Pretty funny moment which lead to this amazing screen shot.
All in all it was a great performance of an iconic song from the Y2K Generation.