Not a classic book, but Artemis Fowl. Disney managed to confuse fans of the books and newcomers to the series alike by adding a McGuffin that was unnecessary, bringing the antagonist from the second book into the movie on the first book, and mangling the relations between the two main protagonists beyond recognition.
Artemis Fowl was just bad, not just a bad adaptation. It was an incoherent mess. Felt like they left way too much on the cutting room floor for the finished product to make sense.