There is absolutely no scientific evidence to support the doctor's assertions that someone devoid of pain sensations would go insane as a result - people suffering from leprosy commonly lose the ability to feel pain or other touch sensations as their nerves deteriorate but this does not in itself cause any psychological damage.
During the scene in the lab when Peyton and his assistant discover that the synthetic skin is photosensitive, they alternate using the microscope though neither of them re-focuses the lenses. No two people have exactly the same eyesight and given the magnification used, they would almost certainly have to adjust the lenses for their own eyes after they use it. Yakitito also keeps his glasses on while looking at the microscope but this could be down to his excitement at the discovery.
In the Chinese restaurant scene, Darkman (as Durant) holds a match under his left hand, showing a large ring on his third finger. Other shots of his hands show no ring, but it reappears later in the scuffle outside the restaurant.
When Peyton removes the bandages to reveal his Pauly face, the dagger tattoo is on the wrong side. It should be on his left side, it is on his right side. (And no, this has nothing to do with the mirror he is looking into.) It is very obvious since the real Pauly is lying in the bed just behind him for comparison. In the next scene at the restaurant, the tattoo has mysteriously migrated back to the correct side.
When Julie follows Peyton back to the warehouse, she sees his "Peyton" mask sitting on a desk. In a close-up of the mask, it starts to melt while still on the desk. Yet in the next shot, Julie is seen holding the mask in her hands.
When Peyton returns to his lab after its destruction, a photo of him and Julie is lying on the floor. The picture has half of Peytons face missing. Later, when Peyton moves into the abandoned building and puts the photo into the machine, more of Peyton's face is visible.
After Darkman disguised as Durant knocks down the real Durant in front of the restaurant, his disguise begins to fail. As Darkman runs off, Durant is seen slapping Guzman in the face. In the next shot, Durant slaps him again.
When Peyton is smashing things with a pipe in the warehouse (while screaming "I'll give you a show!"), he calms down after seeing his reflection in water. He drops the pipe (we even hear it land), yet when we see his reflection again, he is still holding it.
Westlake is viewing the synthetic skin cells with a light microscope, so when the power goes out in the lab the cells would no longer have been visible. Even if the laboratory lights were off to begin with, the cells would have failed to remain stable due to the fact that the microscope operates by the refraction of light through the sample.
Peyton does not use the correct type of microscope to look at the piece of nose skin. To see the cell as he did, a slide of the skin would have had to be prepared and viewed under a transmission electron microscope, and he appears to use a compound light microscope.
When Durant and company are destroying Westlake's lab, Durant is seen turning the valve on a tank of flammable gas to let the gas out. However, he turns the valve handle clockwise, which would shut the valve, not open it.
At the beginning when Durant drives up to the warehouse his cars have black Michigan license plates. Black plates with white letters were changed in 1983 to blue plates with white letters until 1993 and then only the shade of blue was changed after that.
Peyton's funeral is held the very next day after his apparent death despite there being no body to bury or even any confirmation of him actually dying. Regardless of the fact that it would take longer than 24 hours to arrange a church funeral, there would also need to be some evidence that he was actually dead before he could be legally declared as deceased and have a funeral carried out.
When Durant is cutting off the fingers of Eddie Black with the cigar trimmer there is no blood on either the trimmer, Black's or Durant's hands. Clearly there would be huge amounts of blood from the removal of fingers.