Visible crew/equipment: When Terri is attacked by JP, after she knocks him, there's a shot of her scurrying towards the door. At the bottom left of the screen, you can see a boom mic quite prominently for almost two full seconds. (Seen in the 16:9 Blu-Ray from Arrow Video. Some versions of the film appear to be somewhat cropped, so it may not be visible in all versions.)
Revealing mistake: When Joey gets "cut" by the chain in the hospital in the beginning, pay attention. In the shot where she gets "cut," you can plainly see all that happens is the chains slap a little blood on her ankle without any signs of actual damage. Then, a few shots later, there's a very quick closeup of the "cut," and you can see that all they did was paint a little gooey "blood" over the top of her pantyhose, which aren't ripped through at all. (Nor is there a visible cut, either).
Visible crew/equipment: When Joey enters the nightclub at the beginning of the film, you can see the camera's shadow pass over the doorman on the right side of screen as it follows her inside. (Bonus: The doorman also looks directly into the lens for a second.)
Revealing mistake: When Pinhead kills Sandy, his chains attack her, then they lift her up, and her stunt double is visible because her hair is much shorter and her face is completely different.
Deliberate mistake: The film was supposed to be taking place in New York City yet you only see NYC twice when Joey is riding the city bus and another view of the Twin Towers during another scene. Yet is obvious she is not in New York City after running from the Boiler Room and being chased by the Centobites. Even the police cars and uniforms are not NYPD they say Summit Police. Also, if that kind of destruction was going on there would be several more than only two cruisers and three cops.
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