Episode #9.2 - S9-E2
Revealing mistake: Rose's newborn baby, Isabel, is suspiciously large and mature.
Grantchester (2014)
1 revealing mistake in season 9 - chronological order
Starring: James Norton, Morven Christie, Robson Green, Tessa Peake-Jones
Episode #6.4 - S6-E4
Character mistake: The character pretending to be Lt. Ellis refers to himself as First Lieutenant. That rank is referred to orally as "Lieutenant", not "First Lieutenant."
Episode #7.5 - S7-E5
Trivia: The Matron is called Fletcher. Louise Fletcher played the chief nurse (and a similar character) in "One Flew Over the Cuckoo's Nest."