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: During the showdown in the kitchen, Billy and Stu are stabbing each other in the side. Billy gets knife happy and starts stabbing Stu too much, but only in his side. Then when Billy tells Stu to get the gun, Stu turns around and and has a wound on his upper back with lots of blood, but there is no way that could have got there.