Near the beginning, Albert pours himself a glass of milk, drinks over half the glass and sets the glass down on the table. In the next shot, the glass is full again. In subsequent shots the volume of milk in the glass and the milk bottle inexplicably change.
In the initial scenes of the village, an advertisement for War Bonds can be seen on a wall. Although the French government sought funds from its population during the First World War (through National Defense Loans), it would not have occurred in the Second World War as France was so quickly defeated and occupied. The advertisement in this movie therefore, is more likely to be a near-subliminal appeal to the American population to purchase War Bonds to support the US effort.
The iron bar of Albert Lory's prison cell flexes when he pulls away as Professor Sorel is executed.
When Albert and Louise tell the students to rip out consecutive numbered pages in their school books, such as pages 15 and 16, they rip out two pages. In this example, pages 15 and 16 would only be one piece of paper, the two pages on either side of the paper.
The German tanks seen at the beginning are actually American M1917 tanks which are based on the French Renault FT-17 light tanks from World War I. The Germans captured from the French in 1940 and used them for security and airfield roles.
The German troops are all wearing WWI-style helmets, presumably because the actual style of helmets used by Germany in WWII were not available to the film makers. There are also various other mistakes with the German uniforms.
When Paul Martin is trying to escape by jumping from car to car in the rail-yard, one of the parked box cars to the side clearly has the Great Northern logo. Whilst Great Northern was a large operation, its rails didn't reach to Nazi-occupied Europe. Another car is clearly marked "SP" for the Southern Pacific railroad.
After the initial credits, there is a notation "Somewhere in Europe ..." All the signs and notices are in English, so there is no specific country identified this way. The film could be set in Belgium, Denmark, Holland, Norway or any of the other occupied territories. However, when Professor Sorel, Louise Martin and Albert Lory examine the school textbooks for what must be removed, all the references are to French history and literature, squarely placing the film in France.
Railroad freight cars and steam locomotives are of North American style .... not European.