Other mistake: At the beginning of the film a bomber is crashing. As this is happening another bomber is stationary in the sky.
Other mistake: Towards the end, when the Germans are searching for explosives around the guns, there are several Germans using metal detectors, yet they are walking along the steel rail tracks and around metal shells and ammo boxes.
Other mistake: Throughout this movie it is stated that Mallory can speak German "perfectly", that is, without accentual or grammatical problems at all. But Gregory Peck's 'German' throughout the movie is obviously less than perfect. (I ought to know; I live in Germany and speak both English and German well enough to notice the difference.)
Other mistake: Close to the ending you see that the guns are located in the mountain are on the port side of the ships coming in. During the end scene, notice that the burning hole is on the starboard side of the ships.