Revealing mistake: After Joey runs from the club into the street and a car swerves to avoid her, it hits a lamppost which causes sparks; but look closely and you can see that the sparks appear just a split second before the car actually hits the post. The sparks were rigged and set off a second too early.
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).
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.