The Holland scenes ostensibly take place in April, 1945. By that time, most of Holland had already been liberated by the Allied forces. There is mention of British forces being defeated at Arnhem, which was in Sept, 1944, and the subsequent preparation for Hitler's birthday would mark the date as close to April, 1945.
During the gala party held at German headquarters, the drummer is playing a natural wood-finish Premier (British) drum set manufactured in the 1980's-1990's. A period drum set from the mid 1940's would probably have been a white-pearl finish and the drum sizes, hardware and positioning would have been altogether different.
In the scene where Ellis is barely conscious after an insulin overdose, she struggles to find her handbag and knocks over a tub of British 'Players' cigarettes looking for chocolate: in the next shot she is munching the chocolate, but the tub of cigarettes have righted themselves.
After removing the resistance doctor complains that the chloroform used for anesthesia was dated 1941. In fact chloroform is chloroform forever unless it is reacted with something else; the nonsense of basic chemicals in inert bottles "expiring" is a billion dollar marketing ruse.
A torch is used to cut the hinges off of a metal door during the rescue scene. When the door is removed, it is handled right next to the cut hinge. In reality, the metal would be so hot it would be impossible to handle without serious injury.
The goof items below may give away important plot points.
When at the end of the movie characters Rachel and Smaal are pursuing Akkermans lying in a coffin in the hearse, the scene shot at the dyke reveal trees in fall foliage, i.e. they are browning, meaning that this scene was shot sometime in late September or during October. This part of the story is set in the month of May when, in 1945, the actual liberation of this part of Holland took part.
Two German naval personnel, (Bruno Dorfer and Rainer Beck), were court-martialled for desertion and executed by a German firing squad, on May 13, 1945. While German military tribunals could discipline their troops, after having surrendered to the Canadian forces, (under "Allied Military Standing Order No. 153"), Canadian military law should have prevailed, (with no death sentence for desertion, and with a delay of three months before any such execution). The execution of the SS officer, by way of showing a Canadian officer a German "death warrant" and citing "Article 153," was a fiction. However, the misinformation may have been a deliberate lie by the character who presented it; the Canadian officer who believed it and acted on it is later referred to disparagingly.