Revealing mistake: In one scene just before the attack on a big Hunter the young captain (Jürgen Prochnow) is sitting with his charts. Beside him he has got a glass of beer standing. The sub lurches heavily right and left in the hard weather, but the surface of the liquid stays the same.
Other mistake: When attacking the convoy, the crew rushes forward during the crash dive. Just after a crew member jumps through the hatch after the bow crew compartment, you see someone standing still a second before getting pushed in the view of the camera. (01:31:15)
Continuity mistake: At the end of the first attack by the destroyer - where she misses the U-boat and throws her bombs further away - the boatswain makes 22, then 23 strokes on the slate of the Control Centre,thus counting the number of depth charges used against the boat. A few seconds later, in a shot from another angle, with the slate in full view, we see only 22 strokes. (01:03:30)
Plot hole: After the first attack on the U96 the captain decides to resurface after sunset. However when they do resurface it's broad daylight.
Other mistake: The 'Fire Drill' will be a wet business for Lieutenant Werner - the right side window on the car is open. At least you see a horizontal line almost halfway down on his side window. You see it when they meet the Bosun's Mate (time code 1). When they meet the men at their Fire Drill, (time code 2) it even looks as if Lt. Werner opens his window. (00:02:35 - 00:03:15)
Other mistake: During the heavy storm which lasts for 3 weeks, you can see clear blue sky above the U-Boat.
Other mistake: Prochnow drives to the boatyard in the opening scene - the drunken crew at the side of the road initiates him by pissing on the car as it passes. His passenger had just rolled down his window, when he saw the crew, but there is no evidence that he rolled it up before passing the crew. (00:04:55)
Answer: It's the 'Erzherzog Albrecht Marsch', composed 1887.