Other mistake: When Sidney is preparing to go to Tatum's house, she brings several items of clothing downstairs, including a pair of boots, which seem to be knee-length from the way she is holding them to her chest. However later when she is dressing after her session of nooky with Billy, the boots have changed to ankle length. [The clothes that Sidney packed were in the house and she never got them since she left the house after she was attacked and besides the day that she made nooky with Billy was a different day then the day she was attacked.]
Other mistake: After Billy Loomis and Stuart "Stu" Macher are revealed as the villains, they stab each other multiple times to appear as victims. Billy cuts Stu too deep, and Stu begins feeling weak and bleeding profusely from his wounds, showing that he may be dying. When Sidney (in the Ghostface costume) stabs Billy with the umbrella, Stu's rage takes over and he suddenly has the dangerous energy to battle Sidney in the living room. Stu nearly kills Sidney, until she pushes the TV on his head. If Stu was dying in the kitchen, how does he have all that energy to fight Sidney?