A classic on my flight today. It got a bit bumpy, the seatbelt light came on and one of the cabin crew on the PA "...the Captain has turned on the seatbelt sign. Please would all those using the toilets or floating around the cabin return to their seats..." Fortunately it was only minor turbulence so her words didn't come true.
Having said that, 10% of the passengers were probably out of their seats when the sign went on (fairly full 744), so it's easy to see that a sudden upset could catch 40-50 people.