When Friday and Gannon are informed their suspect was found dead in a vacant lot, they depart to check it out. Friday narrates that it's "Friday, 8:17am." When they arrive Friday narrates that the deputy coroner "...estimated the time of death between the hours of 2am and 4 am Saturday morning." When they inform the victims relatives they learn the victim was last seen the night before which would've been a Thursday so the time of death would not and could not have been estimated as Saturday.
When Friday and Gannon are questioning the parking lot boy at the French restaurant he describes the suspects as "Tall skinny, short stout and an average guy." He then say "Tall skinny says to short stout 'Ricki, give this boy five...'" When Friday and Gannon arrest the suspects Ricki is the tall skinny one.
One of suspects has a driver's license in which his first name is only indicated by the initial "J" with no middle name; unless a person could prove that the initial was his legal first name (highly unlikely), no municipality would ever issue a license such as that as the documents would be virtually useless as any form of identification device.
Friday and Gannon collect a picture of each of the missing women. At the end when they match the pictures to the photographs that Negler took of his bound victims, each woman is wearing the same outfit as in her portrait.
Early in the film Friday attends a security briefing for a visit by a Soviet dignitary and notes that the meeting included several Russian "NKVD" agents. The NKVD was actually abolished in 1946 and its foreign security duties were eventually reorganized into the KGB in 1954, over a decade before the film is set.
The trailer-park manager tells Friday and Gannon that Don Negler is currently living in the park. They get to his space--which is vacant. The tenant in the adjoining space says Negler had left maybe fifteen minutes before. This is highly unlikely; it would take him quite a long time to disconnect the utilities and unhook the trailer from its mounting in the space, and hook it up to his car--and he would have to do it without attracting the manager's attention. On top of that, he would be doing it in a pouring rain--and at night.
Carol Freeman was married but her husband was killed in Vietnam, it is very possible that she went back to her maiden name.
Two composites of the murder suspect are drawn by a police artist. Only one of the composites ever matches anyone, and it matched BOTH the jewelry salesman who was murdered AND the murderer of the four young women. No mention is made in the film of how the same drawing matched a dead man and a live man on two different cases.