Despite an abdominal gunshot wound and an exit wound out his back, there are no bullet holes or blood on Richie's jacket or overcoat.
There's a blue mailbox immediately to the left of the front window when Leonard arrives at and leaves the shop, but when Mable leaves the shop, the mailbox is missing.
At the beginning of the film, while he is narrating the steps in constructing a suit, the shots change from overhear, to side, etc. From angle to angle, his wristwatch (worn on the right arm) appears, and disappears.
The Recording in the briefcase is a cassette tape. The movie is set in the 50s and cassette tapes only came out in 1963 in Belgium.
When Mabel phones the La Fontaines in the flashback sequence, she uses a black telephone. It's British, a standard GPO phone. Worse, it's from the 1960s. Presumably it was because the film was shot in the UK.
During the movie, the character Monk racks the action of his pump shotgun twice. The first could be excused as putting a shell in the chamber, but when he does it a second time without ejecting a shell, it's clear that the shotgun is empty.
There are a few occasions throughout the film where it is snowing fairly heavily yet the amount of snow on the glass roof section of English's shop remains the same from start to finish.
Throughout the film people can be heard ringing a bell on the front door of English's shop, but you can clearly see when English enters and exits the shop that there is no doorbell fitted to the door.
At the end after Leonard finishes Francis you see Leonard looking down on him with a fresh clean shirt while blood was gushing everywhere when he stabbed him. A few seconds later in another shot you see bloodstains on Leonard's left shoulder that weren't there before.
At time 1:05:10 of film. It is inconceivable that Roy's bodyguard did not do anything against Francis after Francis put the gun against Mable head, to stop Leonard, when he threatens to talk the truth about Richey disappearance. The bodyguard is looking directly to Francis and should be seeing for his behavior that he was involved in a trap.
"The Outfit" is set during December 1956. Although portable cassette-tape recorders were released by 1955 (e.g. the crude Mohawk Midgetape) and the fully working cassette-tape system shown is either the Minifon Attaché introduced much later in 1959, or the upgraded Minifon Hi-Fi.