This can definitely be a good "bunk science" heuristic along with things like the laws of thermodynamics: If someone is proposing something that seems to break one of these laws/theorems, they'd better (a) realize it, and (b) have a pretty convincing way around it, or some amazing experimental results.

(Note, my work involves magnets and so probably see more than the normal number of crackpot inventors with magnet contraptions).