Hiroshi Oshima, the Japanese ambassador to Germany, played a big part in losing World War Two for the Axis. He had sent a series of messages home to Tokyo, describing seemingly every military secret that Hitler could possibly have wanted to keep a secret. Detailing the strengths and weaknesses of the German defenses along the northwestern coast of Europe must not have seemed like a big deal to him because his message was encrypted. Unfortunately for him, his messages were deciphered. Later in the war, Oshima again unknowingly revealed to the Allies ore crucial information that helped them win the war.
Oshima made several mistakes. Firstly, he trusted in the security of his code. The fact that he was so sure that his communications were secure made him reveal information that he might have otherwise kept more closely. The second was that he didn't try to make life any harder for the American code-breakers working on his messages. His messages were a "wordy, effusive, somewhat emotional, meticulous description of German fortifications along the northwestern coast of France, from Britany to Belgium and everything in between." (pg. 297) As anyone who's ever tried to decipher a chunk of ciphertext knows, the more ciphertext you have to work with, the easier your job is. By being both wordy and specific, Oshima gave code-breakers a gift: they could find out what he was saying because he said so much, and what he said was immensely helpful to the Allies in the war effort. The information gained from Oshima, and from other Axis communications in Europe gave the Allies a leg up in the war, and led to the success of their D-Day invasion of Normandy.
Mundy, L. (2017). Code Girls. New York, NY: Hachette.