Jenifer and Dave are scratching messages to each other on opposites sides of a painted glass. If there was paint on both sides of the glass, they would only see paint, and not the message on the other side of the glass.
While wearing a longer flowered skirt, Jennifer is seen ripping up a black mini skirt with a studded silver belt. In subsequent scenes she switches between wearing the flowered skirt and the black mini that she previously destroyed.
Shortly after she takes off the gauze on her face and rips off the fake wound on her cheek, a spot of smeared blood is on her chest. In the following shot, she goes to retrieve something from the metal slot and the blood disappears.
In the acid shower torture scene, the clothes of Jennifer Tree are changing twice. In the 8th close-up she's wearing all of a sudden a different dress (the dress from the teeth pulling scene). After that it's changing back to the original dress.
Jennifer orders an Apple martini in the night club, when you see her drinking it although it is in a martini glass, the rim is salted. Martini's generally do not come with a salted glass, but Margaritas do.
Near the end of the film when Jennifer grabs the shotgun and points it at her captor, there are no shot shells on the spot on top of the gun, only empty holes. When Gary grabs it and points it at her, red shot gun shells are clearly visible on top of the gun.