The slip-up is in the getting ready for the Prom part of the movie. The guys are tuxedo shopping, the girls are in the beauty shop and Carrie is trying on make up in the drug store. One of the girls in the beauty shop is Norma (with her signature baseball cap sitting on top of the hair dryer - no mistake that it is her). The scene then jumps to Chris & Norma in the Bates school gym while the decorations are being set up and Norma has her hair in braids. Unless Norma has an identical twin that we don't know about or unless she is crazed about rehearsing her Prom hairdo, this is a continuity slip-up.