Continuity mistake: The front door changes in the interior view of Jackie's house a couple of times.
Continuity mistake: At the beginning of the film the teenagers are playing volleyball, Sarah has shorts on. Then she has on only a bikini bottom. Then she has shorts on. Then she has the shorts in her hands and picks them up. Later, Jackie and Diane are talking and in the background, Sarah has her shorts back on.
Revealing mistake: Ken is killing Diane by stabbing her with the knife. You can see that Ken is only stabbing the floor next to her.
Continuity mistake: As the pizza girl walks along the street, she walks past the same pick-up truck three times.
Continuity mistake: Throughout the movie, the light in the foyer is always on, but through the window in the front door there's no light on.
Continuity mistake: When Sarah is walking down the blind alley to get to her car, the wall in the background changes from black to grey.
Continuity mistake: While the girl was in the bath, she got electrocuted, and the plug "fried" and turned black, and part of it even got melted. When the other girl comes in later, the plug is now new and intact.
Revealing mistake: Diane has a bloody wound on her left hip, before the killer actually cuts her with the drill.
Visible crew/equipment: As Janine re-enters the living room, a woman in a white T-shirt and jeans is visible in the room behind her.
Plot hole: When Sarah gets into her car she turns on her radio right away without turning on the car. Don't you need to start the car first in order to turn on the radio?