Factual error: U-571 was sunk on January 8, 1944, by an Australian Sunderland. She was previously damaged by an Allied destroyer on March 22, 1943, but managed to come back to the base. All supply submarines (Type XIV or Type VIIF) were actually sunk by Allied planes or surface ships.
Character mistake: After accurately depth charging U-571, the German destroyer crew suddenly turns out to be almost comically and unrealistically incompetent at the end of the movie. Not only do they barely hit U-571 with their guns, but they also can't keep up with the sub. But even with both submarine diesel engines running flank speed (of which U-571 only has one badly damaged engine operational), a common WWII destroyer would easily be twice as fast as a Type VII U-boat. The destroyer could swiftly close the distance and could either ram U-571 or cross the sub's wake to bring all its weapons to bear and make it more difficult to get hit by the sub. Of course the destroyer doesn't do that but stays perfectly in U-571's wake for an easy kill shot from its stern tube. In reality, it was considered an incredible feat to sink a destroyer with a torpedo, since they were agile at full speed, could easily dodge torpedoes and had shallow draft. This destroyer crew however, seems actively trying to get killed.
Factual error: The language the American soldier and the secret-service officer use is grammatically correct German, but the pronunciation is very bad. No born German would ever be fooled by it.
Continuity mistake: Just after the sub surfaces and the crew arrives on the conning tower, the sub's deck is totally dry.
Factual error: The "German" airplane which spots the crew before the destroyer scene is actually a Fiat G59 - a post-war two-seat training development of the G55 Centauro fighter. It is powered by a Rolls Royce Merlin engine.
Factual error: Although the type of mission depicted in the movie did occur, it was not Americans that were known to do them. Canadian and British teams were specialized to do this mission.
Continuity mistake: After the melee with the destroyer above the surface, they attempt to dive and travel under the keel of the destroyer to escape the deck guns. As the camera is going underwater, the destroyer can be seen, but when the camera is below the surface, its keel cannot be seen.
Continuity mistake: When the German destroyer is with U-571, look at the ship. Notice in the first shots there are 2 gun turrets on either side. Yet in the later shots during the final battle there's only one turret. In addition, right before the turrets are shown turning to fire, look at the ship again - they are already aimed at the U-boat. I guess the Germans felt the need to do this two times.
Revealing mistake: When the U-571's crew member starts firing the MG-34 at the survivors in the rowboat, you can see the ammo belt is all blanks.
Factual error: In reality, the planned boarding of the disabled sub would have been doomed to fail once the American and German subs met. Whenever German U-boats or ships met at sea, they were required to exchange a secret signal, usually consisting of a challenge-and-response using differently colored signal pistols. These signals changed daily and the Americans had no way of knowing this information.
Factual error: After the Americans have stormed the German sub, they find the Germans tried to destroy the Enigma code books by throwing them into the bilge water, which is just below the deckplates in the torpedo room. However, there was actually a lot of room for machinery and reserve torpedoes under the deckplates, which means that the torpedo room would have to be flooded by more than 3 feet deep to even reach the deckplates. This means the whole torpedo room would have been flooded with at least 6.000 gallons of water, rendering the boat likely completely unstable, unfit to dive and overflooding the batteries in the room next to it, which released poisonous chlorine gas when in contact with sea water. Not to mention that this much water could only have been caused by a major leak, but neither Germans or Americans seem to be particularly concerned by any leaks or by the amount of water in the boat.
Factual error: When Tyler asks Tank "what's the status of the torpedo tube?" His hand is on the periscope tube and it spins freely That tube has many seals on it and is stiffer then that.
Revealing mistake: At the end of the movie, when the aft torpedo blows up the ship, the pieces that fly into the air hit the water and there is no splash. Obviously computer generated.
Revealing mistake: The German destroyer is actually an old converted minesweeper.
Character mistake: Near the beginning of the film, when Tyler calls the crew of the S-33 to quarters, look at the guy two back from Mazzola, on the far right. He has his hands clasped behind his back while he's supposed to be at attention. When Tyler says "At ease," he realizes his mistake, drops his hands to his sides where they're supposed to be, then clasps them behind his back again as he stands at ease.
Continuity mistake: In both scenes when the sub is being depth charged, how come the lights keep coming on even though you see them blowing out?
Continuity mistake: When the plane flies over the U-571, an overhead shot shows Tyler and Mazzola looking up at the plane while Trigger and Rabbit wave from beside the deck gun. Then Tyler orders the men to wave at the plane.
Factual error: Several times the movie shows two torpedoes side by side, fired off simultaneously. This was not possible in German U-Boats, a simple timer prevented this and delayed firing the second or more torpedo by a few seconds. Torpedoes in WWII were prone to premature detonation and without this mechanism, one premature detonation could ignite a whole salvo, a significant risk of killing the sub that fired them.
Continuity mistake: When the Americans are first diving U-571, the Chief recommends "closing main vents". Tyler runs over and starts spinning two large red wheels. Each time the scene cuts back to him, he is spinning the wheels in a different direction. Left than right, than left again.
Factual error: Submariners are trained to always listen to the sounds of the boat, since anything out of the ordinary could mean death. Given that training, the small size of a German U-boat and the less-than-skeleton crew aboard it is not possible that the sounds of the fight in the forward torpedo room, or the sounds of the German commander operating the chain on the torpedo loading rail would have gone unnoticed or uninvestigated for so long.
Answer: The scars you see on Tank's face are fake, pure makeup. They wanted the character to have a gritty look.