During the Vietnam War, North Vietnamese intelligence units sometimes succeeded in penetrating Allied communications systems, and they could monitor Allied message traffic from within. But sometimes they did more than that.

On several occasions"the communists were able, by communicating on Allied radio nets, to call in Allied artillery or air strikes on American units."

That is just one passing observation (at p. 392) in an exhaustive history of American signals intelligence (SIGINT) in the Vietnam War that has just been declassified and released by the National Security Agency.