When the creators completely miss the main appeal.

Artemis Fowl - The books' core appeal is that the protagonist, Artemis Fowl, is a bad guy and criminal, but in the movie he's presented as a heroic character.

All Engines Go - Thomas and Friends stood out from other anthropomorphic railway stories by having the trains be treated as realistic engines (aside from the faces) working on an authentic railway, with most of the original book stories lifted directly from real life. The reboot treats the engines as cartoons.