Other mistake: After JP gets the pistons shoved through his head, there's an incredibly puzzling insert shot where we see him crumple to the floor. Suddenly, there's smoke coming off his head, his pants are different, his belt has disappeared, and the pistons on his head are randomly much shorter. It's almost like a snippet of an unfinished effects shot was randomly inserted into the scene.
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.)
Other mistake: When Joey is fleeing down the city streets, two cop cars drive up. Instantly, between cuts, the cops appear outside their cars as Joey runs up to them. They don't get out... they're just suddenly outside of the cars between cuts, and the movie tries to cover it by adding in a car door opening/closing sound.
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