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.
When the storekeeper fills the box with groceries, an overhead shot shows him adding plenty of apples. In the next close-up shot when he adds chocolate bars, there are fewer apples, and a big white-green box that wasn't present before.
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 Citron, his wife and daughter are in the car on the beach, there are no wheel trails in the sand.
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.
In the opening shot of Flame and Citron, taking place in 1944, the street shot shows a 2008-style of streetlight rather than the streetlights current in that wartime period.