Revealing mistake: When JP gets the two pistons shoved through his head and we see them "break through" his head, you can tell it's just two different takes of the same effect, since both come out ABOVE his ear, whereas when we see the final result, there's one coming out BELOW his ear. (The hairline on the fake prop head is also a little wonky.)
Continuity mistake: When Terri goes back to JP, when he's talking about "getting this out of the way", Terri's hands go from down by her sides to nervously up by her mouth between cuts. Yes, her back is to the camera in the first shot, but you'd absolutely see her arms move, and they don't.
Continuity mistake: When Terri goes back to JP and they're talking about the girl who "helped" him with the Pillar, when JP says, "You know what I mean, right?", Terri goes from facing him to facing away between cuts.
Deliberate mistake: When Joey is watching the tape of Kirsty Cotton, right as the scene ends, the TV screen cuts to static. However, when we see the final shot of Joey, we can see the TV screen reflected in her glasses, and it's not showing static, but rather a picture of Elliot Spencer. It seems to be a shot taken from earlier in the scene that they laid in at the end.
Revealing mistake: When Pinhead is in the statue talking to JP, you can see where the makeup ends around his mouth/lips during his close-ups, especially when he spits out the bullets.
Continuity mistake: Right after Sandy's skin is ripped off, there's a shot of the camera moving towards her face as she screams. (Right before Pinhead yells.) When this happens, you can see the chain around her left shoulder fall off right before the shot cuts. It's back on her shoulder in the next shot. It happens quickly, but it's super noticeable if you pay attention.
Continuity mistake: When the statue comes alive and kills Sandy, as she's lifted from the ground, the statue's mouth is suddenly open, but it's closed in the shots before and the shot after.
Continuity mistake: When JP sits down next to Sandy and says, "Welcome!", his cigarette goes from in to out of his mouth between cuts.
Audio problem: When the passerby is telling Joey and Terri that the gallery has been closed, he pulls his little dog away, and it makes a horrible yelping and then growling sound. But the dog isn't making those noises, and they're just overdubbed to add a laugh to the scene.
Continuity mistake: When Joey and Terri arrive at the Pyramid Gallery, after the knock, Terri's hand goes from her side to pressed against the door between cuts. Her cigarette also goes from her mouth to in her right hand in the same cut.
Other mistake: When Terri is cooking breakfast and burning everything, there are multiple shots showing a ludicrous amount of smoke coming out of the toaster, implying she's burnt the toast inside. When Joey pulls the bread out, she pulls out two almost perfect looking pieces of toast - brown with just a tiny tinge of burn around them. Certainly nothing that could cause smoke to pour out of the toaster. (Joey also reacts as though the toast burns her fingers, so it wouldn't be crumbs burning up.)
Continuity mistake: When JP gets his hand bitten by the rat, after he throws it away from him, there's a close-up of his hand covered in blood. In every shot before and after, there's little to no visible blood on his hand.
Continuity mistake: When Joey and Terri are talking and the puzzle-box in the statue gets brought up, in the reflection on the window, you can see Joey begin rubbing her neck with her right hand. When it cuts to the opposite angle, her hand is no longer up by her neck.
Continuity mistake: When Joey gives Terri her cigarettes, when Terri asks, "You gonna have one?", her hands go from on to off the ashtray between cuts.
Continuity mistake: When Terri and Joey are talking about how Terri doesn't dream, throughout the scene she's carrying a cup of coffee. But at a certain point in the scene, her coffee cup just completely vanishes. There's no point where she sets it down. It's just there... and then it isn't. It's entirely possible she set it down in a deleted shot, but the way the scene is edited, it's all portrayed as one fluid conversation with no pauses, so it's a mistake.
Continuity mistake: When Terri is talking about how she doesn't dream, she walks around Joey's living room. The scene cuts to a different angle, and she flops down into the chair. However, before the cut, she wasn't close enough to the chair to sit down. Additionally, Joey's hands change positions between cuts.
Continuity mistake: When Terri goes to Joey's apartment and talks about how she doesn't have dreams, as she stands up, her hands go from far apart to close together between cuts.
Audio problem: When Joey is awakened from her dream about her father's death, she picks up her phone, but the sound effect of her phone being pulled off the base doesn't quite match. (It plays the sound of the receiver fully being picked up before it's fully picked up.)
Continuity mistake: When Joey meets JP, he offers her a rose, but when Joey walks away, it's no longer in his hand as it was in the previous two shots.
Audio problem: When Joey is talking to the DJ, her line "I heard she might be JP's girlfriend," her lips don't quite match what she's saying.
Chosen answer: It's a piston. 2 piston rods are jammed through his head and they move powered by something unknown. You can see the crank shaft and part of some sort of cylinder. A piston is part of an internal combustion engine. It's part of his cenobite form since he liked cars.
lionhead