Mummies, Zombie, Skeletons

Mummies, Zombie, Skeletons? Some sorcerer had fun with magic and decided that the bodies of those who had died should come back among us. Necromancy brings them back to life, but rarely does everything go smoothly, there are heads that don't work, decomposed bodies, limbs that fall apart, and not only that.

Are all necromancers clueless or is necromancy an inaccurate magical science?