The scene where the German soldiers mock the corpses of Flame and Citron is based on how when the Germans took Flame's body out of the house, they reportedly cheered at the sight of the corpse. The soldiers then dragged Flame downstairs by his feet, bumping his head into the stairs continuously.
Near the beginning of the film, there is a montage showing, among other things, a resistance fighter tied to a post and being executed. A square white piece of paper is attached to his torso as a target. Immediately after he is shot we see four bullet holes in the paper forming a "parallelogram" pattern. Moments letter, in a close up of the dead fighter, the bullet hole pattern is reversed. It's pretty obvious that the paper had been taken off between scenes and then reattached backwards relative to the first shot.
Citron picks up a British Sten SMG in the basement scene and fires it. A few seconds later he has a wooden stocked sub machine gun instead (the Sten had a steel framed stock) which looks like a Finnish Suomi SMG.
When Hoffman's drivers are held down at gunpoint in the rain, you can see that the street is dry several yards behind the action and sunshine in on the background buildings. Obvious use of overhead sprinklers.