Every possible consequence of Newton's theories were tested through many experiments and nothing could prove he was wrong. Also, law being a law doesn't mean our understanding of the universe will never change -- it did change with relativity theory.
And it can change again. Naturally most people will take relativity theory to being the ultimate truth, while it is still a theory, not all consqeuences were tested. Although unlikely, it is still possible to prove it wrong, then someone else comes with a new explanation to the problems the theory tried to explain.