Continuity mistake: When Alice opens the pantry door to find Crazy Ralph in there, you can see Ned and Marcy running to the kitchen. It cuts to Ralph, then back again and the other two run into the kitchen again.
Continuity mistake: When the two teenagers find the axe in the bed, the axe is perfectly clean despite the fact it had been shoved through a girl's head. Furthermore, it also looks brand new with a fresh coat of polish on it as well.
Deliberate mistake: When they are playing strip monopoly, Alice purchases Baltic Avenue and then Bill rolls the dice and Alice exclaims that he's rolled an "8" - putting him on Baltic Avenue. However if you look carefully at the dice (before Brenda quickly scoops them up) you can see they show two 5's.