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).
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.
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.
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.
Audio problem: When the cranky lady at the pet-store demands "Where's the manager?!," the audio and her lips don't quite match.
Revealing mistake: When Julia first encounters the resurrected Frank, as Frank speaks, you can see where the makeup ends around his mouth, and see nice, normal flesh inside his cheeks a few times, even though he's meant to be a gross, rotting shell of a human.
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.
Audio problem: When the moving man says "That your daughter?" to Larry, the audio doesn't match the lip movements in the slightest, revealing the line was dubbed. (Several characters were redubbed with American accents in post. Presumably this is the reason for the inconsistency).
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.
Revealing mistake: When Frank's skin is skewered by the hooks in the beginning, it's pretty clear that the skin is just a thin sheet of flesh-colored latex with some fake blood and "meat" behind it. The skin is far too thin and "stretchy" and seems way too "detached" from the tissue behind it.
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.
Revealing mistake: When the cenobites appear to Kirsty, you can unfortunately see where Pinhead's makeup abruptly ends around his mouth, just behind his lips, as he speaks.
Continuity mistake: The blood smears on Kirsty's shirt change slightly between scenes from when Kirsty is in the attic to when she is walking outside later after fleeing. It's the same shirt and the same day, so it makes no sense why they'd be different.
Audio problem: When Kirsty first encounters Frank, he says "It's Frank! It's uncle Frank!" When he says "uncle Frank," the audio and his lip-movements don't quite match.
Continuity mistake: When Kirsty first sees Frank, he grabs the man he's killing and pushes him down to his waist-level. The camera cuts, and suddenly the man is back up and Frank pushes him down to his waist level a second time.
Answer: Letting the moviegoers know what happened to him, he was no longer human, but a piece of meat to be butchered.