Other mistake: When the Principal's son is yelling at his dad to help him with "Jack o'lantern, especially the eyes," he hadn't even killed Billy yet. So how did he need help with a "Jack o'lantern" that had yet to be provided? He was still struggling with him in the backyard.
Revealing mistake: After Steven Wilkins disguised as a vampire kills the unnamed woman he closes her eyelids to make it look like she's asleep. Just before his hand contacts her eyelids you see her almost blink, revealing she's very much alive.
