Factual error: One of the US Marine Corps guards in the government building (in 1941) is wearing the single rank chevron of a lance corporal. This rank was not used in the USMC between the 1930s and 1958.
Factual error: Chief Superintendent Hilton wears the ribbons of the General Service Medal (awarded for military service in a combat zone, most commonly in Northern Ireland) and the Police Long Service and Good Conduct Medal (awarded for twenty years' service), but not the Golden Jubilee Medal (awarded to all officers with five years' service in 2002), which he would also have. When he reappears in Series 4, he is wearing the Golden Jubilee Medal, Diamond Jubilee Medal (awarded in 2012) and Police Long Service and Good Conduct Medal, which would all be correct, but no longer has the General Service Medal.
Caleidoscope - S2-E4
Character mistake: The chess set in Dr. Pierce's home office is set up incorrectly. When the men are set up properly, each player has a light colored square in the right-hand corner. The board as it appears in the show is set up so that the light-colored square is in each player's left-hand corner. This "beginner" mistake puts the king and queen in the wrong positions.
Visible crew/equipment: When Rebecca Madsen jumps in her car to pursue the armored truck, as she does a U-turn and takes off at high speed, you can see all the people standing on the side watching the filming. Some even have their phone cameras out videoing it. (00:29:30)
Other mistake: Kellog gets tagged by Kiera in an earlier episode, and as a result, he's shunned by the gang (and booby-trapped) because she can track him to their location, and they have to find a new hideout. In this episode, he finds the gang's new hideout, and nobody seems to care about the tracker anymore.
Factual error: On the missing poster in the hospital, one of the gendarmes is said to hold the rank of Brigadier. This is a Police Nationale rank equivalent to sergeant; it is not used by the Gendarmerie Nationale, since all ordinary gendarmes are equivalent in rank to sergeant. The lowest supervisory rank in the Gendarmerie is Maréchal de Logis-Chef (staff sergeant).