Other mistake: As the Germans open fire on the British sailors in a life boat, there is a panning shot of the crew. In this shot, a hair (or scratch) that made its way onto the film itself is visible on screen. (00:27:20)
Continuity mistake: Just after unveiling the Nazi plates on the SS-33, there are 3 seamen wearing brown jackets next to the deck gun. When the shot changes there are 4. (00:25:00)
Visible crew/equipment: Just as the SS-33 sets off we see the crew on board say "all ahead two thirds". As they say this the camera moves behind a ladder resulting in a camera shadow on the ladder. (00:20:50)
Continuity mistake: During the mission briefing, the position of the Top Secret folder changes from the centre of the map to the side of the map between instant cuts. (00:23:30)
Continuity mistake: Tyler states he doesn't know what's going on to a few seamen and tells them to do their jobs. As the shot changes the seamen have all disappeared from the background. (00:14:50)
Continuity mistake: When Tyler is out on the deck, he has his hand raised. When the "boat secure" order is shouted it's now lowered and holding on to his glass. (00:12:10)
Factual error: When the German "destroyer" is attacking, it drops a steady stream of depth charges. I reality, depth charge attacks were made by locating the sub on sonar and dropping a pattern of five or ten charges at a time when the escort ship was above the target. These were set for the estimated depth and would have gone off at roughly the same time.
Plot hole: The Americans disguise S-33 as a German Type VII U-boat to pose as the German resupply boat. This is a pretty bad idea, since all resupply boats are of the Type IXV and not Type VII. Of course in a severe storm the differences between types might go unnoticed, but the plan could not rely on that alone. In good visibility, any able German watchman would be able to spot the difference quickly.
Continuity mistake: After the German captain has attacked Tank and handcuffed him to the crane, the pattern of blood on Tank's face changes at least twice. First, he has two channels of blood running down the left side of his face under his nose and on his chin, and no blood on his lips or the right side of his face. Then, when Eddie and Tyler arrive, there is blood smeared all over Tank's nose and lips. Finally, after Tyler takes the cuff off and is yelling at Tank, all of a sudden there is a line of blood from Tank's nose across his cheek to his right ear. (01:09:05)
Continuity mistake: Just after Mazzola's been killed, Tyler unlocks the handcuffs from Tank. The handcuff starts swinging as soon as Tank pulls his hand free, but in the next shot it's stopped moving. (01:09:42)
Deliberate mistake: When the Americans are in the U-571 battling the resupply sub, Tank gets the engines running and starts the sub moving forward. There is an exterior shot of the sub's propellers, with the port prop turning counterclockwise and the starboard prop turning clockwise. Then Tyler wants to turn right to heading 010, and he orders "Starboard back full", which should reverse the direction of the starboard propeller to help turn the sub. Two exterior shots later, however, the props are shown still turning exactly the same way they were before Tank reversed the starboard engine. (00:52:00 - 00:53:35)
Continuity mistake: Mazzola has just been killed and when Tyler helps Tank, first he holds Tank using his left hand, then the next shot by his right.
Answer: The scars you see on Tank's face are fake, pure makeup. They wanted the character to have a gritty look.