Toward the end of the movie, Burt Lancaster and Clark Gable are watching a Japanese submarine that they just torpedoed exploding behind the shallow draft decoy vessel that the Japanese sub was hiding behind.

In the scene where the camera is pointing from the back of the conning tower towards the bow of the U. S. sub, you can see the decoy vessel between Lancaster (on the left) and Gable (on the right). If you look to the left of Lancaster, you will see the conning tower of another submarine following the decoy vessel.

According to the story, there are only two subs in the fight, the one that Gable and Lancaster are on and the Japanese sub that they just torpedoed. There shouldn’t have been a third in the scene.