I read this tidbit about the Hunt for Red October….the movie revealed classified information.
It confirmed the existence of naval technology that was still classified at the time.
In the movie, the Red October can be super-stealthy by turning off its sonar (which can alert other subs to its location) and navigating silently through a method called "gravimetry." In real life, Russian subs didn't have that capability—but U.S. subs did. Oh, and it was a secret. The information was declassified a few months after the film was released; whether that was because the movie had let the cat out of the bag, or because the tech was by then not very new anymore, no one can (or will) say for sure.