Factual error: The moment Nancy's digital watch hits zero, her bedside clock rings waking her up. You cannot set an old, mechanical style alarm clock to go off with anything like that level of accuracy. (00:32:00 - 00:33:18)
Continuity mistake: After the bathtub scene, when Nancy opens the mirrored medicine cabinet and removes the bottle of Sta-Awake, her fingernails are short as usual, but in the bottle's closeup Nancy's fingernails are quite long. (00:34:10)
Continuity mistake: When Nancy leaves the kitchen to go to school, her orange backpack is empty, but when she storms away from her dad after he arrested Ron, her backpack is full.
Answer: The scene you are referring to is a dream sequence. In dreams everything is different and how they change throughout the movie is no mistake.
SAZOO1975