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)
Audio problem: When Steve says "What?" to Kirsty at the pet-store, it's pretty painfully obvious that the line was dubbed in during post. The audio is much more loudly spoken than he appears to be speaking. (Several characters were redubbed with American accents in post. Presumably this is the reason for the inconsistency). (00:45:33)
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)
Visible crew/equipment: As Frank is killing the last victim Julia brings him (the meek man), Kirsty slowly opens a door and slinks through. As she does, in the bottom left corner of the screen, you can see part of a shadow from either the camera or a cameraman. It's definitely a camera/cameraman shadow, since the shadow is wavering in a similar way to the camera's movement. (00:57:12)
Visible crew/equipment: When Kirsty is going up the stairs at the house (right before she finds Frank), at one point, she rounds a corner and walks by camera in extreme closeup... and you can see her walk through the shadow of the camera. (The scene then cuts to an opposite angle, and there's nothing else that would have cast the shadow she walks through). (00:57:24)
Revealing mistake: When Kirsty encounters the 'skinless' Frank, he says to her 'It's Uncle Frank' and you can see his left hand is normal and flesh covered, even though Frank is supposed to still be missing all the skin from his body. (00:58:10)
Visible crew/equipment: When Ashley is running down the corridors before escaping back into her medical room, you can see the dolly that The Engineer chasing her is being pushed on. (01:04:50)
Audio problem: When Julia and Frank are talking after Kirsty first encounters the cenobites, if you look closely, Frank's words don't quite match his lip movements a few times. It's especially noticeable when he snaps out "What I care about is a new skin!," which doesn't match in the slightest. (01:09:15)
Deliberate mistake: When Larry comes inside the house (right after Kirsty encounters the cenobites and Frank and Julia discuss getting him a new skin), he is absolutely bone dry... but we can hear a steady, heavy downpour outside when he opens the door. Seems like they added a rain sound effect in post for a creepy ambiance even though it wasn't shot with that in mind. (01:09:35)
Audio problem: When Steve and the nurse discover Kirsty has fled the hospital, their lines don't quite match their lip movements when they are on-screen. Especially when Steve says "Maybe she's gone back to..." (Several characters were redubbed with American accents in post, which may explain the discrepancy). (01:10:04)
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)
Visible crew/equipment: As Kirsty exits the storage room (where the corpse spewed maggots on her), you can see a boom-mic shadow appear on the wall behind her in the upper left corner of the screen, and even follow the actress slightly as she moves. It's only on screen for a couple seconds and can be hard to catch, but once you see it, it's super obvious. (01:19:36)
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.
Revealing mistake: When Frank is trying to stab that girl, his switchblade is obviously fake because there is a line on it that the dagger will go into when he hits her with it.
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.
Answer: Letting the moviegoers know what happened to him, he was no longer human, but a piece of meat to be butchered.