Passengers on a Singapore Airlines flight were left in a chaotic mess after the plane fell 20 meters when it hit severe turbulence and all this while dinner was served.  11 passengers and 1 crew member were injured on the flight from Singapore to London. Passengers had been warned that there would be severe turbulence and that food service would be temporarily suspended. The captain issued an order for all flight crew to take their seats immediately. When the plane hit the turbulence passenger say that everything, including people not tied down hit the ceiling.  After the turbulence the cabin crew did an excellent job of cleaning up and everything was back to normal in short order. [Source]