When the copycat killer does the "Dahmer" crime, he dumps the decapitated body into the river. Jeffrey Dahmer would've never done this, as he kept all the bodies in his apartment for further sexual gratification, then he dismembered them and submerged them in acid, sometimes he ate parts, also he boiled the heads and kept the skulls for a sculpture he was planning to do. He killed for companion, to have a submissive partner by his side, he killed men he felt attracted to and he would've never gotten rid of a body.
Dr. Hudson incorrectly says that Peter Kürten was a serial killer in the 1930s. Kürten committed his first known murder in 1913 and the rest in 1929. While Kürten did attack several people in 1930, they all survived. He was arrested in May 1930 and executed in July 1931.
When MJ goes to Helen's house and finds a dead officer, she accidentally nudges his flashlight with her foot and it rolls into the wall, leaving a trail of blood behind it. After radioing for backup MJ picks up the flashlight, which is suddenly next to her foot again, and the blood trail is gone.
When Helen looks through her database, the name of the Hillside Strangler victim whose crime scene photos the killer used as model is listed as "Tanya Canseco". No Hillside Strangler victim had that name.
When Monahan comes into Helen's apartment with her gun drawn, she sees herself 'live' on the TV screen, while the camcorder is pointed toward her. The Post-It note reads "Press Play". When a camcorder is in "camera mode", pressing Play will do nothing. In order to play a recorded tape, you must first switch the camcorder to the video playback setting.
According to Helen's database, the Son of Sam victim whose crime scene the killer used as model was named "Janis Fulkner" and she was killed on April 25, 1977. In real life, none of Berkowitz's victims had that name or were killed alone in a car, nor did he even kill on that date.
When inspectors Goetz and Monohan are at the crime scene of the David Berkowitz copycat crime, Helen tells them to go look for a note and inspector Goetz confirms to her that they found it, and that it says "Police, let me haunt you with these words. I'll be back". They then go to her apartment, and there she is totally unaware that there was a note. She says: "What? there was a note, wasn't there? I was right!". Yet they had already told her there was.
In the bathroom crime scene there are two candles burning. M.J. asks the coroner for the time of death, which he claims to be eight hours ago. It is unlikely that the candles could have been burning for such a long time.
The goof items below may give away important plot points.
When M.J. shoots Peter Foley, he falls to the right of a cement strip on the roof with his left hand and gun resting on it. When Helen walks toward M.J., Peter is lying to the left of the cement strip and his left hand (holding the gun) is the part of his body furthest from it.
In the end of the movie when MJ shoots Peter on the roof he falls down with the gun in his left hand. His left hand falls onto a concrete strip. In the next shot, when Helen passes him by and kicks away his revolver, his right hand is on that concrete strip, his left hand (with the pistol) is in the gravel.
In the final scene where MJ shoots the killer in the shoulder, she is seen to cock the hammer back on her semi-automatic pistol. If she'd just fired the weapon the hammer would be back for the next shot, she wouldn't have to cock it.