From a few years ago - an odd sequence of numbers helped find spies

https://www.mattblaze.org/blog/neinnines/

A nice story that goes into some detail about one time pads (OTP) and the radio station in Cuba that did broadcasts of numbers on a regular basis. People (including the FBI) noticed that MANY of the transmissions had no nines in them. This happened for 10 years. The frequency of those helped the FBI catch some Russian spies.