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.
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.
Revealing mistake: When the German destroyer is hit by the torpedo at the end of the film it is seen to stop dead in the water upon the torpedo impact. In reality momentum would have carried the ship forward some distance after the explosion. A thousand or so tons of steel just does not stop on a dime unless it hits something like a mountainside.
Answer: The scars you see on Tank's face are fake, pure makeup. They wanted the character to have a gritty look.