Other mistake: The envelope with the letter for Amanda is suddenly in a very good condition as it ends up in the peanut bowl... even though it has been in the trash, had chewing gum attached to it, been trodden on, had a beer barrel rolled over it, etc....
Other mistake: When the girl throws the brownie at Denise, it is apparent that the chocolate gets into her red hair, but in the next shot when Denise enters the bathroom, the chocolate has mysteriously moved to her cheek and her hair was clean.
Answer: The filmakers decided to forego showing Amanda's face early on to give her grand entrance at the party more impact.