Animation is a big complex process that involves a lot of hands with tight deadlines; "we changed some stuff in the process of revising a scene and did not re-r

Animation is a big complex process that involves a lot of hands with tight deadlines; "we changed some stuff in the process of revising a scene and did not re-render every other scene this change touched because of time constraints/human error" is probably a good theory for most continuity errors, though Teddyh's theory of "automatically randomize the magnet colors" sounds pretty plausible for this particular case. It sounds like something really easy to set up for the props once, on the expectation that they will never be more than background detail, then accidentally draw attention to with a closeup sequence animated by multiple animators. In general, animators will take any shortcut they think they can get away with. :)