As Andrea and Gregg are kissing on the diving board, their position on the board changes between shots.
When we see Terry stand behind his stepfather while he is readying his gun,
he is very close to the door, but in the very next shot, he is leaning on the door.
Terry stabs the assistant in the left part of his torso, but the wound is shown to be on the right.
Assistant's pose changes after death.
Doctor's dead pose is changed when later found by Todd, and her legs appear fully in the shot, unlike the first shot of her dead body.
You can see man's hand twitching after being cut. Human body parts don't have the nerves needed to do something like that, at least not that hard.
Doctor is shown cut in half, but still screaming. Body is too small to be her real body, and is obviously fake, her real body actually being in a hole in the ground allowing her to act as a part of it. Legs are shown twitching farther from her body, something human body parts can't do when cut off, at least not that strong.
When pair is found murdered in sauna, they can be seen breathing.
When stepfather is sitting at the desk, and mother is talking to him, you can see him moving, though being dead.
After mother comes to the boys after Terry killed the guy in the car, we see a shot of him, and you can see his eyes move a little. His pose is also changed from the one when he is killed.
In the beginning drive-in scene, the boy in the blue shirt is the one who finds the ax and walks away with it, but immediately after, the boy in the white and red shirt is shown wielding it.