PG (but actually PG-13)

A movie about adultery is going to get a rating higher than G. But since The Graduate came out in 1967, before the PG-13 rating debuted, it surprisingly has only a PG rating—surprising since there's flashes of brief nudity in the movie, and since it's all about adultery. So, retroactively, it has a PG-13 rating, and has apparently been re-rated by whoever's job it is to care about these things.