Continuity mistake: Right before Wendy sends the email that her husband is dead, a can of shaving cream can be seen on the right side of the sink. After hearing her husband come into the hotel, she knocks over the can of shaving cream, which is now on the left side. (00:47:05)
Factual error: In several different episodes characters are escaping on foot to Canada, as evidenced by Ontario license plates. But the show is set in New England, and the only way to get to Ontario through the Eastern United States is by crossing a major body of water: The St. Lawrence River, Lake Ontario, The Niagara River, or Lake Erie. You can't walk there.
I Surrender - S1-E10
Continuity mistake: When Asha is in the hospital, her hair is pulled back and towards her right, away from the wound and a lot of her hair is next to her right shoulder. In the next shot of her, her hair has changed position.
Episode #2.8 - S2-E8
Factual error: In the opening seconds of the intro the BTK killer is following a woman home in his work van. Both the woman and BTK have license plates on the front of their vehicles. Kansas does not use front license plates. (Also, BTK's van shows the county abbreviation on the plate as WY - Wyandotte County - where Kansas City, KS is located nearly 200 miles away. The county tag abbreviation should be SG for Sedgwick County).
Episode #4.9 - S4-E9
Factual error: Renate escapes from the prison in a Morris Ten Four, with the badge visible on the bonnet. However, this designation was not introduced until 1935 and the episode is set in 1931.
Factual error: When Detective Inspector Sara Lunt's photograph in uniform is shown on the TV news she is wearing a double row of silver lace below her hat badge. Only chief constables wear this insignia, well above the rank of inspector.
Factual error: DI Renton is a Metropolitan Police officer 'parachuted' into Kent to solve a big case. This hasn't happened for many decades. Local police forces investigate their own crimes without help from the Met.
Factual error: The board at the entrance to the abandoned air force base reads "Canadian Aviation Forces" and lower down mentions the "Minister of Defense." There has never been any such organisation as the Canadian Aviation Forces (it was the Air Command from 1975 to 2011 and is now the Royal Canadian Air Force once more) and the minister's title is the Minister of National Defence (without the American spelling).