I can't remember which class it was, but this started happening the first day of class, about 2 minutes from the end. In a couple of weeks, it was 5 minutes, and more people were doing it. A couple of weeks later, it was closer to 7 minutes, and 3/4 of the class was doing it. The professor finally yelled at everybody to stop packing their stuff up and sit and listen until the end of class.