The failure to appreciate how paranoid and scared the Soviets were almost started a nuclear war in 1983 with the Able Archer debacle - at least Reagan realised that the "Evil Empire" rhetoric was a mistake and calmed things down.
[NB I'm not trying to defend the Soviets, who were a thoroughly unpleasant bunch, but the colossal failure to understand their motivations and just how one side the Cold War was for most of the time almost ended things.]