Not R rated, but I did see the 70s Lord of the Rings in the theatre.
A pretty unlikely thing to have happened, as I'd have never heard of it had I not read the books (and I was very young when I did that), it was a midnight showing which my parents let me see by myself (yeah WTF in retrospect) and it wasn't like that movie got a lot of theatres showing it.
I saw it in the movie section of the paper and could not believe that such a thing existed.
I fell asleep halfway through and thought I missed the RotK part. Lol.