I have a couple of pet peeves when it comes to movie adaptations of novels, particularly novels I read and loved as a child: the Earthsea miniseries, The Seeker: The Dark Is Rising, and The Black Cauldron.
In each of those adaptations, the makers seem to have taken a couple of elements and decided that they could do better with the rest themselves. And, in my opinion, they failed miserably.
The changes aren't just updating the storyline or elements to suit a new audience or a new medium: they change the entire story. For example, in The Dark Is Rising, the nature of the Six Signs (or five in the movie, I believe) is changed completely. to the extent that the makers probably knew they'd never be doing the sequels because they wouldn't work with what they did with the Signs.