The Cleve Cartmill Affair

Science-fiction author Cleve Cartmill is best known for his short story "Deadline," which appeared in the March 1944 issue of Astounding Science Fiction. The story isn't famous because it's a good story. Cartmill himself described it as a "stinker." It's famous because it contained specific details about how to make an atomic bomb — details which Cartmill somehow knew over a year before the existence of the bomb had been revealed by the U.S. government.

When Military Intelligence learned about the story they freaked out, fearing a security breach, and launched an investigation. They questioned Cartmill, as well as Astounding editor John W. Campbell. But eventually they concluded that Cartmill had gained his info, as he insisted, from publicly available sources.

However, the rumor is that Military Intelligence bought up as many copies of that issue of the magazine as they could, to prevent its dissemination. Which makes that issue quite valuable. For instance, at Burnside Rare Books it's going for $400.

But if you simply want to read the issue (and Cartmill's story), you can do that at archive.org for free.

More info: lynceans.org, Wikipedia.