In the opening scene (after the credits), Elle's hair is in bangs. In the scene where they discuss the killer using zip ties instead of ropes her bangs have suddenly grown in.
After identifying a suspect, they say that the suspect lost mobility in the right side due to a spinal cord injury. When we see the suspect with the victim, he only uses his right arm, never his left, as if his left side is injured.
At 21:16- The double-sided masking tape used to hold the fake duct tape gag can be seen.
When Walter Kern is holding the gun on the final woman, just before he is captured, he points the gun and tells her to lie down on your back or he will kill her, there is the sound of a gun cocking, but the gun is not cocked.
PA drivers license use 8 numbers and no letters in them. There is no electronic signature under the expiration date, and the website on the left is not correct.
During the close-up of the Pennsylvania driver's licenses of the two victims (one who was murdered in 1987, and one that was just murdered) both licenses are digital. However, Pennsylvania didn't switch to digital licenses until the mid-1990s.
The serial killer gives the final victim a set of plastic handcuffs and tells her to put them on. In subsequent scenes it is obvious that the cuffs are still slack and were never tightened on her wrists.