Continuity mistake: Right at the beginning when Frank is with the dealer at a table, a green glass of tea appears/disappears randomly through shots, and they also change position, as does the cenobite box. (00:02:00)
Continuity mistake: The amount of blood on Julia's face, neck and clothes changes from when she's inside the attic to when she goes out onto the landing, and then from when she then goes into the bathroom to wash herself clean. (00:38:05)
Continuity mistake: When Frank grabs Julia's wrist/hand in the attic after she tells him to "Keep your voice down!," watch closely. The way he's gripping her wrist/hand changes a few times between cuts. Additionally, depending on the angel, her hand either is or isn't casting a shadow on her face. (00:43:36)
Continuity mistake: When Frank holds up the puzzle box to show Julia, the box rotates in his hand instantly between edits when it punches in to a closeup. (First, it's the side with the large circle in the center facing out, then it's the side with the much smaller circle facing out.) As the scene cuts back and forth between the two angles, this error keeps occurring. (00:47:18)
Continuity mistake: After Julia sees the vision of Frank being tortured, she walks over to a window and faces it. The scene then cuts to a wideshot, and instantly Julia goes from facing forward, to facing sideways. (00:48:20)
Continuity mistake: When Larry and Julia begin to kiss in their bedroom doorway during the storm, the way they're holding and touching each other changes a few times between cuts. (00:51:28)
Continuity mistake: When Kirsty first encounters Frank wearing her father's skin, Frank says "It's finished" and Kirsty replies "Why is it finished?!" Suddenly, Frank rotates about 90° between cuts. (01:12:30)
Continuity mistake: When Kirsty accidentally opens the box while in the hospital, the face with the large diamond design on it is initially on top, then when it cuts to a close up, the box somehow flips 90 degrees so the face with the circle on it is on top.
Continuity mistake: Throughout most of the film, Larry has a bandage on his right hand. When he talks to Julia through the bathroom door, Larry's bandage is on his left hand.
Continuity mistake: When Kirsty goes into the storage room (where the corpse spews maggots onto her), she has a large bloodstain on her face. Inside, she sweats most of the blood off her face, leaving only a slightly discolored red stain. But then, when she exits the room, suddenly a lot of the blood has re-appeared on her face. I'm assuming they filmed her entering and exiting the door at one time, and her inside the room another time, causing the inconsistency.
Continuity mistake: When Steve finds Kirsty in the collapsing house, they hug, Steve runs to the door and Kirsty notices the puzzle box shifting, the amount of debris falling in the air and the amount of "dust/flakes" on them changes between shots several times.
Continuity mistake: When Kirsty is sending the Cenobites back, when she sends back the female Cenobite, we see a closeup of her hands opening and closing one of the "configurations." When it cuts to a medium shot of Kirsty holding the box out, you can see that the configuration is still "open" in her hands despite having "closed" it in the closeup.
Continuity mistake: When the female Cenobite is dragging her knife along the wall and says "Not leaving us so soon, are you?", in the wideshot she's not leaving a mark on the wall at all, but then it cuts to a closeup, you can see that a large rip has appeared on the wall between cuts. (And there's also blood coming from it for some reason).
Continuity mistake: When Frank is torn apart by the Cenobites during the climax, the moment is incredibly oddly cut. In the first shot, he's being held in place by the chains, but then there's a closer shot, and he's suddenly already torn apart and is flying into pieces. (Evidently, the MPAA made them cut a few seconds out of the shot, leading to this odd error).
Continuity mistake: When Kirsty is scooting along the floor of the storage room, a few shots before the corpse falls down and spews maggots on her, you can see her shirt is pulled up rather tightly around her neck. Then suddenly, right before the corpse falls out, it's pulled down a bit again. (Because they wanted the effect of the maggots falling into her shirt).
Continuity mistake: Throughout much of the third act at the house, the smeared makeup under Kirsty's eyes subtly changes a few times, and even disappears and reappears once or twice. Since she's crying and sweating throughout the sequence, it seems they were reapplying the makeup every so often, and it sometimes didn't quite match. (It happens enough that I think one "blanket mistake" is best, rather than pointing out every single incidence).
Continuity mistake: When Kirsty is fighting Frank in the attic, as she grabs the puzzle box, watch closely. It's very subtle and hard to notice, but the box rotates about 45° between cuts when it punches from the medium shot to the closeup of Kirsty grabbing it.
Continuity mistake: After Larry stumbles into the attic, he bleeds a pretty large puddle onto the floor. He then stumbles towards Julia, and in the wide high-angle shot, we see no blood on the floor whatsoever, even though it should be visible.
Continuity mistake: When Larry and Julia are checking the house at the beginning, Larry flicks a switch, which illuminates a statue with red lights. In the wide shots where Larry is in front of the statue, the light on him is totally normal and there's also plenty of space to his right. In the medium-profile shots of Larry, he's bathed in red light, and there's a wall only a yard or two to his right.
Continuity mistake: When Frank solves the puzzlebox in the beginning, right before the chains shoot out, there's a wide and then a closeup shot of Frank looking down. In the wide shot, his head is only very slightly tilted down, and he has a somewhat blank look on his face. When it cuts into the closeup, his head is suddenly tilted down a good 45°, and he has a more intense look on his face.
Answer: Letting the moviegoers know what happened to him, he was no longer human, but a piece of meat to be butchered.