Factual error: When the UBoat changes course, Andrews, operating the radar repeater, is immediately able to provide the bridge with the target's new course and speed. This would be impossible without extensive calculations and at least range and bearing plots once the target has steadied up on the new course, either figured on the scope screen or on a maneuvering board. One must remember that both vessels' courses and speeds must be taken into account when making computations, and the relative direction of a target's travel most often is not its true course.
Character mistake: When the U-Boat first appears on the U.S.S. Haynes' radar screen, operator Andrews turns his head to the screen a split-second before the blip actually appears. The swiftness of his reaction indicates that he is reacting to the radar signal, not that he has his face turned to the screen by chance; the actor simply made his move too early.
Other mistake: Near the beginning of the movie, at the same time the ship makes radar contact, the sub acquires a signal on its oscillator. The sub captain thinks it's a false echo due to the rough seas. He so advises the next man that comes on duty but tells him that if the signal appears to get any closer, "you will awaken me immediately." The captain isn't awakened until the "dive" alarm sounds after the men in the conning tower make visual contact with the ship. Wouldn't the oscillator signal closing in on the sub have warned them long before they made visual contact?