I was on a flight this afternoon when the captain turned on the seatbelt sign in light turbulence approaching Zurich. Shortly afterwards a woman a few rows down stood up and opened the overhead bin and a bag containing some alcoholic duty free items fell out and hit another passenger on the head and shoulders, no broken glass or permanent damage but it looked fairly painful.
These items may or may not have been dislodged by the turbulence but, nevertheless, a good example of why you should stay seated and belted once the sign is illuminated.....