Deliberate mistake: In the scene near the end of the movie when Trigger is in the bilge trying to stop the air leak, the concussion of an artillery shell fired from the destroyer shakes some pipes loose. The mistake is that one of the flanges visible on a section of pipe has no bolt holes, making it impossible to join that pipe to anything.
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.