There's a pretty thorough list here. It's happened like 12 times in the last 7 years, so fairly rare I guess. Some of the movies it happened with barely got a theatrical release- the comedies "Yoga Hosers" and "Barely Lethal". I guess the most notable case was last year when Clint Eastwood's movie "The 15:17 to Paris" got arbitrated down from an R to a PG-13.
It's kind of hard to tell what the "first" movie to have this happen with was. The PG-13 rating's only been existence since 1984. From what I can tell from this list, the first movie to get arbitrated down from an R to a PG-13 was the Mel Gibson movie "The River" which came out in December of 1984. This was followed by the movies "Mask" and "Desperately Seeking Susan" in 1985.
Before that, you had movies that managed to get their rating lowered from an R to a PG like "Poltergeist", "Tootsie", and "Terms of Endearment".
Weirdly, the MPAA's website (filmratings.com) doesn't even mention an appeal for "Mean Girls", the movie Rebel cited. I do remember on the commentary track on the DVD Tina Fey talking about how they had to change some things to get a PG-13- changing "have you had your cherry popped?" to "have you had your muffin buttered?" (the latter seems DIRTIER, to be honest) and also "masturbated with a hot dog" to "made out with a hot dog".