Archives of fanfiction on the net have traditionally grouped stories according to rating (i.e. X, R, PG-13, PG, and G), so that everyone knows what to expect before they read a story. But it turns out that their use of the rating system may be illegal. A few fanfiction writers have apparently begun receiving cease-and-desist notices from the MPAA demanding that they stop using the rating system since it's the intellectual property of the MPAA.

MPAA sending C&Ds for improper use of rating symbols