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)
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: 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: 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)
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)
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 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).
Audio problem: When Chatterer appears and shoves Frank back in the attic, we get a closeup of his teeth chattering, but when it cuts to a wideshot, he's no longer doing it. You could argue that he just stopped as the camera cut (a little flimsy, but arguably a valid correction), but the real issue? They continue to play the "chattering" sound effect for about a second after he stops even though his teeth are no longer moving. They try to mask it by fading it out quickly... but it makes the moment feel wonky.
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 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).
Revealing mistake: As Frank (in Larry's skin) pulls the knife out of Julia's stomach, we get a quick closeup of the wound. If you pay attention, some of the blood appears to be unnaturally thick and "goopy" and is "rolling off" of her shirt rather than sticking to it and staining it, revealing it's fake. A lot of fake blood recipes cause this issue... some will stick and stain the fabric, but a lot of it will kind of just roll off because it's sometimes thicker than actual blood.
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: 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).
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)