In all the years that I've traveled there has only been one flight that I actually screamed and grabbed my best friend's leg (left a massive bruise too!)
Flying home from Florida at night, we passed through a bad thunderstorm and hit one of those crazy air pockets and the plane instantly dropped mid-air about 25-30 feet. The lights went out for a few seconds, things got tossed around only 1-2 people had bumps/bruises since they didn't have their seatbelts on.
Afterwards we laughed but I really thought we would die that night