At the ~13 minute mark, after Mae walks past Joe in the Doyle house (Joe: "It's your life." Mae: "Yes, that's what's so funny. It's really mine."), you can see the moving shadow of one of the crew at the bottom left of the screen, on the floor.
When Earl the projectionist is rewinding the recently run roll of film (around 10 minute mark), he uses a manual crank re-winder from reel to reel. One of the reels is wrong because he is putting the outer film side on the inside of the new reel.