Revealing mistake: When Kirsty encounters the 'skinless' Frank, he says to her 'It's Uncle Frank' and you can see his left hand is normal and flesh covered, even though Frank is supposed to still be missing all the skin from his body. (00:58:10)
Revealing mistake: When Frank is trying to stab that girl, his switchblade is obviously fake because there is a line on it that the dagger will go into when he hits her with it.
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.
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.
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.
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.
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.
Revealing mistake: When Frank is solving the puzzle-box in the opening scene, you can pretty blatantly tell when they switch out the prop, because little lines, gaps and shadows appear on the box that weren't there before. (Since it wouldn't be possible for a single prop to do all the different "functions" of the mystical box.) It's most noticeable when the first moving part appears, and you can see several hard "lines" that weren't there in previous shots.
Answer: Letting the moviegoers know what happened to him, he was no longer human, but a piece of meat to be butchered.