Flavorwise posted a very interesting article about the most inappropriately rated films since the first appearance of the "PG 13" rating 30 years ago, August 10th 1984, which was given to the "Red Dawn" movie (ironically, also dubbed the most violent films in history).
I all heartedly agree with the following sentence, excerpted from a review written by the late Roger Ebert, about the 2000 Cameron Crowe film "Almost Famous" :
"Consider, Coyote Ugly, which glorifies girls who dance on top of bars to sell more drinks, gets a PG-13 because there is technically no nudity. But Almost Famous, which shows a bright teenage boy successfully negotiating the minefield of a rock tour and forming a value system with the support of his mother, gets an R because of brief and insignificant nudity and language, and drug use presented as a cautionary lesson. If you were to see the two movies side-by-side you might be as mystified as I am why the MPAA thinks one is appropriate for 13-year-olds, while the other is questionable for 17-year-olds. But of course the MPAA cannot have values; it can only count beans, or nipples, or four-letter words.”