Hmm, in that case, it might be relevant to point out examples that don't quite fit Plasmon's situation but are almost the same: There are a variety of examples where due to a lack of rigorous axiomatization, statements were believed to be true that just turned out to be false. One classical example is the idea that of a function continuous on an interval and nowhere differentiable. Everyone took for granted that for granted that a continuous function could only fail differentiability at isolated points until Weierstrass showed otherwise.