This year marks the 20th anniversary of the MPAA's notorious NC-17 rating, a designation whose checkered history continues. It all began when, for some reason, the MPAA failed to copyright its "X" rating, which recommended that no children under the age of 17 be admitted. Regular, even prestigious movies could earn an X rating, like Midnight Cowboy and A Clockwork Orange, and no one would blink an eye. But then the porn industry (legally) stole the X and started using it as a marketing ploy, even going so far as to invent the "XXX" rating, for (presumably) extra-naughty movies. Years later film critics like Siskel & Ebert, recommended an "A" rating for "Adult," which would come somewhere between the "R" and the "X," but the MPAA -- in their infinite wisdom -- came up with the NC-17 (no children under 17), which was the equivalent of the "X."
The full article is quite interesting, and breaks down a list of the most notable NC-17 films of the last two decades. (Tanking at the box office is a commonality most of the films share.)
I feel that most of my own spec scripts would just graze a light R-rating, were they to be produced. Maybe. What do you think the hardest rating a piece of your writing would garner?