Deliberate mistake: When Greg gets murdered and Cindy, Bobby, and Buffy are trying to figure out what happened, you can see the killer is mopping up the blood. However the floor is carpeted so the blood wouldn't clean up, it would smear instead. (00:31:20)
Deliberate mistake: When Cindy is talking to the killer on the phone, and he says, "I could be anywhere. I'm like the wind, baby," she walks into the living-room. Notice the wall behind the couch, it is completely different a moment later. When the killer hides behind the curtains, the wall behind the couch has large bay windows with seating underneath. Then when the killer is at her house the second time, all the windows are gone during the Matrix send-up, so Cindy can run along that wall.