Andrew takes Sam to the 6:00pm showing of Gone With the Wind, a four hour epic. Afterwards, he's walking her home in full sunlight. In September it would be dark long before 10pm, even with "Double Summer Time".
When Vaudrey is first offered the choice between the cyanide pill and being shot, we hear the murderer cock the revolver. Less than a minute later, when the gun has been shoved in Vaudrey's face, the murderer cocks the gun again.
Throughout the series Routemaster London buses appear. The first prototype of the Routemaster was not built until 1954.
Mrs. Powell says she has been running the boarding house for "about 2 months, just after Dunkirk". As this is set in October then Dunkirk would have been 5 months earlier.
A young black marketeer is shot by a member of the Home Guard whilst stealing from a food warehouse. The Home Guard shoots the thief with a Model 1917 "Eddystone" Enfield rifle, but later in the show, Mr Foyle announces that a body has been found with a "Lee-Enfield" bullet in him. Same caliber, totally different weapon.
Andrew asks Sam to go to the 6:00pm showing of Gone with the Wind after she gets off work. It would have been well after 9:00pm after they got out of the film (and this during the month of October). However, when they get out of the movie, it is broad daylight and not even close to being dark yet.
Sam refers to bird-watchers as "twitchers" - the term was not used until the 1950s.
The poster for The Postman Always Rings Twice (1946) hangs in a garage. The film was released six years after the setting of this episode.
When Foyle picks up Sam from the MTC workshop, there are a pile of tyres in the background that include the unmistakable, exaggerated zig-zag tread patterns and rounded shoulders of radial tyres. These were not invented until 1948.
When Mrs Powell answers the phone and is asked who is calling she says, "They didn't say". The use of "they" as a gender-vague singular pronoun is a very recent change to the language. Back then she would have said "He didn't say" or "She didn't say" as appropriate.
Foyle's son, Andrew, says in this episode that he was eight years old when his mother died. But the year is 1940 and other episodes establish that Andrew is 22 in 1940 and that his mother died in 1932, making him at least 14 at the time.