On an overnight flight from SFO-SYD last year. Cabin was dark, nearly everyone was asleep - myself included. So far the flight had been very smooth. Out of no where all I remember is feeling my body being nearly weightless for just a second and people screaming. I have no doubt that the seatbelt kept me in my seat.
The pilot came on and re-iterated the reasoning to always where your seatbelt when in your seat. To the best of my knowledge, there were no injuries. I didn't get back to sleep for a couple hours after that event.