For president, 1952. Adlai Stevenson was nominated on the third ballot at the Democratic Convention.
For vice president, 1956. Estes Kefauver was nominated on the second ballot at the Democratic Convention. He defeated JFK, who was ahead in the roll call. However, Sam Rayburn, who chaired the convention, kept the balloting open, allowing delegations to switch their vote to Kefauver. (There's actually YouTube video of the roll call.)