The inspector played by Sam Neill fails to remove his hat when he is in the Roman Catholic church. Altho there are already Great War memorials (1919) admirably with devotional candles burning - there is no eternal red oil lamp to show the eternal presence.
When Grace and Tommy are talking and he offers her a cigarette, they are filtered, even though filtered cigarettes weren't invented until the 30's and not popularized until the 50's.
Ada is in a cinema eating popcorn, but popcorn and other snacks weren't eaten in the cinema until after sound was introduced to films in 1927 which was eight years after the episode takes place.
"A Charleston" is asked for at the dance in 1919; "Charleston" the song was not written until 1923, and the dance did not become a fad (in the USA) until 1925.