This is not very nice of me to say, but I think part of the reason I wondered about Herbert’s mental state at the time the Lynch film was released was that 1984 and 1985 are when the last two Dune books came out and as you’ve already heard me say, they’re terrible.

Chuck Palahniuk agreed that the movie of Fight Club was better than his book (well duh) but I guess that wasn’t really a mangling.

Philip K Dick died about three months before Blade Runner was released, and visited the Blade Runner set and made no secret of the fact that his perspective on the replicants differed greatly from Ridley Scott’s, but I think he said he really liked the visuals he got to see, so I’m guessing he may have leaned positive on the movie had he lived to see it…?

Stephen King, whose opinions of movie adaptations of his work cannot easily be trusted, liked The Mist, which as you probably well know totally changed his book’s ending.

I grew bored trying to look up other answers to this question, so let’s just go with Dune for now.