Senfeld S5E9: Because Elaine is dating a man with the same name as New York serial killer Joel Rifkin, she wants him to change his name to a less embarrassing one and suggests O.J.; about six months after this episode aired, O.J. Simpson was arrested and charged with the murder of his ex-wife and her friend.
There was also an episode of Buffy that was held from airing because the subject was a potential school shooting and it was planned to air like the day after Columbine.