The Daughter of Autolycus - S4-E5
Other mistake: Lady Felicia says "Ah, Father, how very propitious." The subtitles say "Ah, Father, how very capricious." (00:03:10)
Father Brown (2013)
1 other mistake in season 4 - chronological order
Starring: Mark Williams, Nancy Carroll, Alex Price, Sorcha Cusack
The Sins of the Father - S4-E9
Factual error: A character is revealed as having been hypnotised into committing murder against their will. This is not possible. (00:05:20)