Factual error: Michael's impending 18th birthday is often mentioned as the time he becomes an adult and can make his own decisions. The age of majority in 1922 (and until 1970) was 21.
Other mistake: The Birmingham police officer, Moss, is still listed in the credits as Sergeant Moss. He held that rank in Season 1, but in Season 2 he wears inspector's rank insignia. Campbell does address him as Mr Moss, the correct title for an inspector.
Continuity mistake: The second man to die in the ploughed field "execution" scene had the back of his head blown away by a gunshot. While the first man had the back of shirt covered in blood. In the replay in next episode, his head and the shirt show no such effects.
Other mistake: When Polly tells Ada about her two children being taken from her, she says Michael was 5 and "Sally" was 3. When she goes to the seance she says her daughter would be 18, but shortly after that Tommy finds Michael and he is 17. Also, when Tommy tells Polly that he found the records on her children, the daughter's name changes to "Anna."
Suggested correction: The age difference could definitely be the case. Depending on the timing, she might be 18 years 9 months and he 17 years 1 month; in a month, Sally would turn 19 while Michael would still be 17. About the names: Since the children were taken from her, it would make sense that they changed the girl's name from Sally to Anna (I'm not sure how Polly reacted). Because Michael was already five, a name change wouldn't have been necessary - he was old enough to remember his original name.
Factual error: Major Campbell heads the Irish branch of the secret service, but reports to Winston Churchill, the Colonial Secretary. The Colonial Office had nothing to do with Ireland, which at that time was part of the United Kingdom.
