Aldrich Ames

A high-ranking officer in the CIA's counter-intelligence division, he was unmasked as a spy who had been paid several million dollars by the Soviets to provide top-secret information to them. It turned out that he had given them, among other things, the names of Russian civilians and military personnel who were working as spies for the CIA, all of whom were eventually imprisoned or executed by the Soviets. Convicted of espionage against the United States, he was sentenced to life in prison without possibility of parole.

Historians don't really like to carry on speculative debates, but you could certainly argue that the likelihood of a Soviet invasion of Western Europe was extremely, extremely low.