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(at around 20 mins) When Tom returns to the adult video store for the second time, immediately after approaching the counter, a boom mic briefly enters the shot in the upper left.
On the Missing Persons card, Mary Anne Matthews is spelled with a "E" in Anne. It is also spelled this way in the credits. However, on the thank you card to Tom Welles from Mary Anne's mother at the end, the girl's name is spelled without an "E" by the mother.
When Dino says to Machine to hold down Tom Welles in the storage, Machine takes at the same time a knife in his (left) hand, but in the following scene, when Tom resists and shouts at Dino, we see once again Machine taking a knife from the table, in order to threaten Tom again.
The black car that Tom drives has a license plate in some scenes, but does not in others.
When Tom Welles first meets Janet Mathews, he is looking at pictures of Mary Anne when he comes across a young picture of Janet. During the close up shots of the photographs, the picture behind the photo of Janet shows Mary Anne in a white shirt and green dress, while in the expanded shot of Janet and Tom, the photo shows Mary Anne in a blue shirt.
The car Tom leaves the "studio" in when running from Eddie and the one he arrives at the cabin in are not the same car. The first car has caps over the lug nuts of the alloy wheels and the one at the cabin has the lug nuts exposed and the wheels are a different style.
To ascertain Machine's identity, Tom calls several emergency rooms, pretending to be a police officer, asking for Machine's real name, insurance information, and home address. Even in 1999, no hospital would ever give this information out over the phone and would need an in-person request with a court order to be in compliance with HIPAA laws (which were first passed in 1996).
During Tom and Max's first encounter, Max states that "If you're an undercover cop and I ask you if you are, you have to tell me." This is a widely believed but untrue misconception.
Max mentions "foot fetish films" when he's talking about the most extreme, sickest kinds of porn. In reality, foot fetish films are considered probably the least offensive, with many of them not even having nudity or intercourse.
Tom Welles' transportation of his firearm throughout the film is at odds with reality in many ways. While it is legal to travel by air with a firearm, it must be done in a locked case that is inspected by airport security (even in the pre-TSA days), then stored with checked baggage. Any security personnel noticing a NFA item, in this case Tom's suppressor, heading for a state where they are prohibited (CA and NY in this case) would certainly suggest to Tom that this is a very bad idea, and a felony. In order to do this legally, Tom would need to file a NFA Form 20 and have very specific firearms dealer and manufacturer licensing, which it is extremely unlikely a random PI would have.
Further, while it is likely that Wells, as a private investigator, would have a carry license in his home state, it is extremely unlikely that a PI based on the east coast would have any privileges in California, and largely impossible that he would have such in NYC. It is unlikely an experienced PI would risk losing their license, and a lengthy prison term, to transport a suppressor around the country just in case they happened to need it.
Further, while it is likely that Wells, as a private investigator, would have a carry license in his home state, it is extremely unlikely that a PI based on the east coast would have any privileges in California, and largely impossible that he would have such in NYC. It is unlikely an experienced PI would risk losing their license, and a lengthy prison term, to transport a suppressor around the country just in case they happened to need it.
Hollywood has no red-light district.
Tom Welles begins by examining the 8mm film on a PC with Windows, but it changes to a Mac.
EDIT: The program Tom uses is a Sony based Corel program, an early predecessor of CorelDRAW, and several other programs. However, in the earlier versions of Windows, certain file extensions had to be opened using Quicktime, which was owned by Apple/Mac. Also, Windows, Mac, and a few other programs, used a GUI based off of a program that was discarded by Xerox, making them all have a similar look and function.
Some projectors, particularly for home use, can introduce a perforated steel plate between the lamp and the film, allowing them to display a freeze-frame.
In the warehouse, Longdale makes Poole kick his gun under the car, and this is the gun that he is reaching for. Some believe that Poole is trying to grab Longdale's gun, but he is really looking for his own.
In the movie, the character's name Janet Mathews is clearly pronounced as "Janet", but in the handwritten thank-you card, her name is written as "Janent".
When Machine slits Max's throat, Max slumps over dying, there was no cut or blood at all.
When Longdale is killed with an arrow, he is holding a gun still. Then Tom gets the drop on Eddie and machine. Eddie then lays on the ground directly beside where that gun would've fallen trying to reach a gun that not only was very far from his reach. But also could've linked him to the scene if he had reached it to shoot Tom.
Welles is casing the house and sees Machine's mother through the window. Close shots of the mother are accompanied by muffled dialogue that does not match her mouth movements.
When Max California is explaining that the music industry is not exactly beating down his door. His is actually saying something else.
At about the 7:04 mark in the lower right-hand of the screen, an inordinate amount of smoke is building up. After it is blown away with the wind, you can see a steady stream of fog pouring out from what can be suspected as a fog machine as the source.
(at around 20 mins) When Tom returns to the adult video store for the second time, immediately after approaching the counter, a boom mic briefly enters the shot in the upper left.