Eragon. They really fucked up so much about the book!

The two adaptations of Watchmen have both missed the point. The Zack Snyder movie treats the characters like gods rather than deeply flawed losers and weirdos.

The HBO series is better, and does get very close, but collapses from a meandering plot and glorifying cops

I'm sure it's not the worst but I felt like the adaptation of Watership Down changed the tone/message compared to the book. Now granted the infamous violence is present in the book (though seeing it is more visceral than reading about it). But in the book there's a nice story at the end where Hazel is injured (iirc) and is taken in by a little girl and her parents who take care of him while he recovers before releasing him back to the wild (which only adds to his legend, of course).

Removing this bit, the only positive interaction with a human, makes the message feel more like, "Humans are bastards and inherently anethma to the natural world, which is also a brutal war of all against all even down to the cutest softest creatures." It just makes you feel bad, whereas the book might make you feel bad at times but it also offers an example of what you can do right. It's kind of a pet peeve when a work with environmentalist themes falls into that line of "Humans are the problem and there's nothing you can do but feel bad about it."

Hunger games.

Well. It's clearly not a book, but Disney's Hercules was the first time I really felt disappointed about going to the cinema. My ten year old brain was having none of it. I wanted the adultery, the murder, the dirty stuff the story was supposed to have and I think it's the Disney film (that I've watched) I hate the most up to date.

The NeverEnding Story

The Black Cauldron

The hobbit

Inkheart by Cornelia Funke is one of my most beloved series

read that series several times and when they announced a movie I was so hyped!

and the movie was just ok :C
I haven't seen any movies based on books since then unless it receives high praises which I haven't seen much

The witcher is pretty bad.

Queen of the Damned was pretty awful and threw out the majority of the storyline.

If TV shows count: Earthsea.

It was such a poor adaptation that Ursula Le Guin wrote an article denouncing it.

Don't know if it counts as "classic", but Mortal Engines comes to mind. The film cut out over half the book. I loved the book and got really excited for the film, but it was a massive let-down. They could've easily made the film twice as long, maybe more.

Not a book and not a movie, but that Cabinet of Curiosities series adapted a couple of HP Lovecraft stories and it was fucking terrible. There were a couple of beats that were interesting, but generally it was very faithless and the changes were for the worse. There were some excellent episodes otherwise, but I can't help but feel that they are just butcherings of much better stories that I haven't read.

Politically, it's way less bad than you'd expect, I'd recommend watching it. One of the best episodes had -- to someone as brainrotted as me -- an incredible hybridization of classic horror and battle anime logic. That one was probably my favorite one, though there was one where the protagonist looks just like the Disco Elysium guy and kind of acts like him too, and it was fun.

P.S. did you know that there are movie adaptations of Ayn Rand's drivel? If you are masochistic, they might be fun to watch

I know its not a classic, but Dan Brown's inferno the book and the movie have two different endings and it angers me every time.