In The Silent Service, Den Beste gives a potent example of how “loose lips sink ships”:
Japanese anti-submarine warfare capability was never really very good, and in the early part of the war it was particularly dreadful. One reason for this was that the Japanese had incorrectly calculated the depth to which American boats could dive. They set their depth charges based on that, and the American submariners soon learned that if they dove deep enough (at depths well within the safety limit of the boat hulls) they would be all but immune to Japanese depth charging.Obviously this was quite useful and interesting, and it began to be talked about back on shore, in a “Boy, have you heard about how stupid the Japanese are?” kind of way. The story spread, and spread, and it eventually ended up in a newspaper. And then one Congressman included it in a speech before Congress.
As might be imagined, the Japanese heard about this, and started setting their depth charges to go deeper. This prompted an officer in the Navy to send a letter to the Congressman saying that he was sure the Congressman would be pleased to learn that the Japanese had corrected their mistake.
Beste points out that our present-day Special Forces soldiers are in a similar, if even more vulnerable, situation.