When the first couple are having sex, they mess with the lights so that they can change the camera angle for a better view. But later, when they're watching the tape with their distributor the angle of the camera is wrong. They're watching a part that happened before they switched to an overhead view, but the angle is overhead anyway.
After Caleb, Tanner and Jessica check into the motel, a scene shows the "no" part of the neon light coming on, but later on when the older couple arrive at the motel, the light is off again.
Flipped shot: Jessica stabs the chair fragment into Smith's left cheek. When she emerges from her hiding spot underneath the building, there is a shot of Smith stalking in the woods and the hole has moved to his right cheek. The film was obviously flipped.
During the scene in the trailer with the killer, it's very clear the packing boxes are empty.