The only time the polling was wrong was in 2000 where Dubya was expected to win by a million and lost by half a million but still got in by getting the SCOTUS to stop the count in Florida.
Dewey was expected to win by a landslide over Truman in 1948 but the final polling was done in September that year Dewey sat on his ass while Truman went cross country to the end. Anytime before 1952 needs an asterisk beside it...