A seat belt test, for sure.
I always have my seat belt on when flying. Always.
I was on a flight once that plummeted, and it dove so sharply that several people (I saw one a few rows up) who rocketed upwards when the plane dropped, and the one I saw smashed full tilt into the underside of the carry on compartments. There was an elderly woman in the head who had to be taken off on a stretcher.
We diverted, and spent the remainder of that flight with soda, coffee, food gravy, and other things dripping off the overhead onto us. (they had just finished serving right before the planed dropped)
They took three people off that flight on stretchers. All the other people? Before the drop, we were all strangers. After the drop, people were just yakking it up with everyone in every seat around them. Interesting how effectively danger can draw people together.
So, yeah...I keep my buckle fastened when flying.