In the scene where Mr. Slausen leaves Molly with the rifle in the truck, you can see a short male mannequin standing behind her in a sort of saloon setting. Thereafter any time you see her from the same shot, he's no longer there.
In the opening scene, a knife sticks in the wall by the victim's head. In most subsequent close-ups, the knife is not there.
Jerry's mummified mannequin corpse was taken apart by Mr. Slausen, with the head shattered into pieces. However, when Molly is driving down the road with the mannequin versions of her friends in the back in the ending, Jerry is suddenly back together.
When Woody jumps through the window, glass is scattered all over as expected. When Becky climbs out barefoot, some of the glass has been broomed away for her to escape.
The amount of plaster on Tina's face.
String is plainly visible when the flying pipe impales Woody, and when the scarf starts to strangle Eileen.