Episode #1.3 - S1-E3
Other mistake: The publican asks for payment for some drinks, saying "One and six, please", meaning one shilling and sixpence. The subtitles say "One-o-six, please."
Episode #3.5 - S3-E6
Continuity mistake: Sidney is attending a pre-wedding dance in the camp he is visiting wearing a light colored shirt. When he leaves the dance floor to chat with other attendees, he is now wearing a green sweater over his shirt.
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 #8.2 - S8-E2
Other mistake: Paulie takes off his blood-soaked shirt at the laundry. His undershirt is clean. It should also be covered in blood.
Episode #8.5 - S8-E5
Other mistake: We are told several times that the "Special Reserve" is kept locked in Mr. Palmer's office. When Anna comes to the bar to "see" it (and put the poison in it), it is in plain sight on the shelf behind the bar, and her husband hands it to her.
Episode #9.2 - S9-E2
Revealing mistake: Rose's newborn baby, Isabel, is suspiciously large and mature.