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)
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 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)
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.
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)
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)
Continuity mistake: After Larry storms away in confusion after Frank skins the rat, Julia's eyes close and her hand moves up to her chest between cuts.
Revealing mistake: When Frank skins the rat in front of Julia in a threatening manner, if you look close, you can see that the actor is simply running the knife through a pre-made cut in the dead-rat prop, and isn't actually skinning it. What really gives it away is that you can kind of see that the skin is already separated at the bottom and even briefly "bends" before the knife reaches it.
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)
Answer: Letting the moviegoers know what happened to him, he was no longer human, but a piece of meat to be butchered.