United Flight 935 (Boeing 777) with 196 passengers, 12 crew and 3 pilots, was flying from Heathrow (London UK) to Los Angeles (USA) over the Atlantic Ocean when it hit turbulence. The turbulence resulted in 4 passengers suffering with fractured bones and 1 flight attendant spraining her ankle. The seat belt sign was off when the plane hit the bump that resulted in the rapid altitude loss. Since the sign was off, there were passengers walking in the aisles resulting in the injuries.
The flight ended up landing at Montreal (Canada) around 1:15 p.m. and aircraft was taken out of service. United Airlines provided another aircraft from passengers to be flown from Montreal to Los Angeles.
So, please whenever you are flying keep the seat belts on!