Jimmy Darmody Has Sex With His Mother (Revealed in Season Two, Episode 11)
To cheat on your girlfriend hours after she drops the bombshell that you knocked her up is one thing, but to do it with your own mother is sinking to new lows that would make even the most dangerous gangster cringe. There was never any question that the relationship between Jimmy and his scandalously young showgirl mother, Gillian, was inappropriate. Whether it was the way she passionately wrapped her legs around her son, recently returned from the Great War, or casually mentioned to his wife, Angela, that when she would change baby Jimmy's diaper she would "kiss his little winky," Gillian's love for her child knew no boundaries. But it was during a flashback to 1916 where we found out how far the line had actually been crossed – when a drunken Gillian initiates incest with Princeton student Jimmy.