We’ve all seen the propaganda posters from WWII about keeping quiet on military operations. But was this a real fear? Was there any instance where Axis powers found out about something critical by listening in to the common man on the street?
Possibly, though it's hard to fully prove. .
House of Representative Member and chairman of the House Military Affairs Committee during WWII, Andrew J. May is not necessarily a common man, but certainly is credited for the loss of multiple US Submarines in the Pacific Theater thanks to a rash disclosure he made to the press. With his role as chairmen of the Military Affairs Committee, May had access to a good amount of classified wartime briefings and the like.
Congressmen May held a press conference after returning from a war zone briefing tour, where he revealed the fact that Japanese Anti-Submarine Forces often set their depth charges to explode at a shallower depth than US Submarines could dive, allowing the subs to effectively go deep and escape.
The press published this information, wiring it to newspapers across the country and ultimately the world, thus alerting the Japanese to their error, and here's where the hard facts end. A number of American published sources (books on the war) have printed a statement by Admiral Charles Lockwood, Commander, US Pacific Submarines, who estimated that this leak saw the loss of as many as ten submarines and some 800 crew.
This seems pretty damning for May, and he certainly got some nasty press over this. However, we have to look into some other details as well. May's leak did not disclose the test depth that US Submarines were capable of, nor does a post-war US Navy report mention any change in deployment of depth charges by the Imperial Japanese Navy after the May Incident. Nor do any of the common sources I have handy contain any information on the Japanese side of the conflict, relating this discovery. Someone else may be able to expand more on this.
Ultimately, we can't prove that May's loose lips actually sunk any given submarine, but the incident proves well why military and government officials were worried about such matters.
Sources:
Submarine Report Depth Charge, Bomb, Mine, Torpedo and Gunfire Damage Including Losses in Action 7 December, 1941 to 15 August, 1945 Volume 1
Clay Blair (2001). Silent Victory: The U.S. Submarine War Against Japan. 1. The Naval Institute Press. p. 397.
Follow-up question: And vice versa, did the Allies gain any intelligence that way in Germany, Italy, Japan or any of the other fascist states?
Additional question. Looking at a lot if the posters from the time it seems like a lot if the posters were referencing cargo shipping rather than just military. Was a lot of it also about convoy movements and the merchant marines? Iirc the Germans attacked a lot of shipping on the east coast.