In Midway (2019), the Japanese were using encrypted traffic which the US was codebreaking. The Japanese were planning an attack on a location they were referring to as AF.
Washington believed that AF was located in the South Pacific.
But Layton denies it:
Layton: I heard that Midway accidentally sent out an unencrypted transmission that their water plant was broken.
Nimitz: And is their water plant broken?
Layton: Not that I know of, sir. But that proves Midway is AF.
What exactly proved that Midway was the location the Japanese were calling AF?