Have you ever seen an airplane propeller that looked like this? It's called the rolling shutter effect {wiki}. A cell phone camera records what it sees by scanning from one side of the frame to the other (or top to bottom), and strange things happen when the scene you are shooting moves faster than the image scanner. Another video illustrates how the slow scanning process of a cell phone camera creates this effect. -via reddit
http://www.fazed.org/view/?id=18128&p=1#L4