When Disney restores an animated film, it has artists inspect it frame by frame. Other studios aren’t so meticulous and rely on digital tools designed for live-action films. From As Cartoons Go Digital, Something Gets Lost:
The technology at issue — called “digital noise reduction,” or DNR — works by removing lines that appear in one frame of a film but not the next, reasoning that the line doesn’t belong. In live-action films, that usually works well. But in cartoons, the process gets sketchier.