The comment citing the mentions in the film of French dying in Normandy as a factual error is wrong. The French and British were engaged in ferocious fighting with the Germans in Normandy in 1940, following the evacuation at Dunkirk. So the references in the film to Frenchmen dying in Normandy are in fact correct.
In one of the last scenes where Michelle Williams is driving away, the camera pans out to a landscape shot. The adjacent wheat field clearly shows tracks of a sprayer used to dessicate the wheat - there was no such thing in 1940.