Morfin Gaunt was first imprisoned for attacking Tom Riddle Sr after he realized that his sister Merope fancied the latter. He was imprisoned for three years in Azkaban. He was imprisoned the second time when he was wrongfully accused of murdering Tom Riddle Sr and his parents. However, this only happened because his nephew Tom Marvolo Riddle (Lord Voldemort) altered his memories and made him proudly admit to a crime he didn't commit.