Continuity mistake: When we see Susan and her friend David laughing, Harold's at the closed window. When Susan comes back into the room the window's open.
Continuity mistake: The actress playing Dolores is suddenly played by scream queen Linnea Quigley. The original was recast due to the actress having problems with the nudity, and shots of her were cut out as much as possible.
Other mistake: As Debbie walks through the foggy maze from her upstairs bedroom to the bathroom across the hall, she closes the door and then asks herself why she is in the kitchen, when the kitchen is obviously downstairs.