Continuity mistake: In episode 2-4 "The Funk Hole " which takes place in the fall of 1940, Andrew states to Sam that his mother died when he was eight years old. In this episode (Feb to June 1941), Foyle is looking at his wife's grave; it shows his mother died in 1932, which would have meant Andrew was born in 1924, and only 16 in 1940, far too young to be a Spitfire pilot. Also, his commanding officer states "the younger pilots look up to you, Andrew", before one of the "younger" pilots (age 19) is badly burned in a crash in Andrew's plane.
Factual error: When Sam reads the newspaper, one article claims that 254 people were killed in a Paris air raid. This episode takes place in February 1941, when Paris was already occupied by the Germans, and England did not bomb Paris. Oddly enough, this same article appears in a previous episode.
Other mistake: In "The Funk Hole", the son of Foyle tells Sam that he was 8 years old at the time his mother died. But in this episode Foyle stands at the grave of his wife, who had died in 1932. So that would mean that his son was 16 when he joined up and became a Pilot Officer - far too young.