Buffy finds the science teachers glasses on the floor, and she puts them on the table. In one shot she places them with the legs folded out right side up, and then in the close-up, the legs are folded in. In the next shot, the glasses are upside down on the table.
When Ms. French is making her sandwich the sleeves on her sweater go from being pushed up around her elbows to down to her wrists between shots.
When Cordelia opens the refrigerator in the cafeteria and sees the teacher, she screams "His head! Where's his head?" however, when they show her face right before she finishes speaking, her mouth isn't moving.
At 4:20 when Mr. Gregory, the science teacher, is looking at slide photographs, he is sitting in front of a light board (used to view slides); yet he tries to view the slides by holding them up in front of him.