Continuity mistake: Every time we see the Nurka fire a torpedo, regardless of whether the order was "fire one," "fire two," or even "fire five" or "fire six," the torpedo is always seen leaving exactly the same tube.
Continuity mistake: In the final battle scene, Lt. Bledsoe is alternately wearing a long sleeve shirt rolled up at his elbow or a short sleeve shirt.
Answer: Sailors are very superstitious, they considered it a bad omen. Number 7 is an area of the ocean where most ships and subs have gone down.