The plastic arm changes position when the teacher sets it on the table in front of the guy. When the teacher is holding it, the hand is facing the left side of the screen, but it's facing the right of the screen when he sets it down on the table.
The college professor should not have taken his students (some of whom are underage) to a bar, as he would have been immediately fired from his job. He also would have lost his job for charging the drinks to the campus and hitting on one of his female students.
A piece of lighting equipment can be see at the far right when the the old guy finds Jigsaw sitting on the swing.