I once saw an R-rated trailer before a PG-rated movie. (The movie was the original "Hairspray"... I don't remember what the trailer was for.) I particularly remembered it because I noticed that there was a family with kids there. But then they left during the opening credits.
Edit: Oh, I remember this more clearly now -- One of the parents (having come, apparently, without any idea what they were going to watch) had asked me in the lobby if the movie was OK for kids, and I said, well, it was rated PG. Then the R-rated trailer came on and made a liar out of me. That's why it stuck with me.