There are 2 types of trailers: red-band and green-band. The trailer you usually see in front of a movie that reads, “This trailer has been approved for all audiences” has a green background and is a “green-band” trailer. There is another kind, the red-band, that is not approved for all audiences because it has something “bad” in it (read: more fun to watch).
In the past, only green-band trailers have been shown. But today, Regal Cinemas announced that they are going to start showing red-band trailers again. And since Regal is the nations largest chain, i have to hope that this starts a trend of it getting nasty before your film begins.