When the guys are talking about the x-rated movie in the classroom, the canister with the school's film is on the table closer to the front and Carvelli's movie is the canister toward the rear. When Horshack reaches to pick up the dirty movie, the close up shows him grabbing the one in the front and not in the back. In the following shot the front canister is still there and the back canister is now in Horshack hands.