1572: Gregory XIII elected pope
Gregory is best known for his calendar reform, but he also promoted Church reform and the Counter-Reformation.
964: Pope John XII
John was the son of the ruler of Rome, Duke Alberic, and was made Pope when he was about 18. He crowned Otto I Holy Roman Emperor, then broke with him after Otto issued orders that the pope take an oath of obedience to him. The emperor replaced John with Leo VIII, who was in turn deposed by John.
Shortly thereafter, Pope John died in the arms of his mistress. He was not yet 30.