Consider a universal prior based on an arbitrary logical language L, and a device that can decide the truth value of any sentence in that language. Such a device has no finite description in L (according to Tarski's undefinability theorem), so the universal prior based on L would assign it zero probability. A human would also think that such a device is unlikely, but not infinitely so. (I gave a version of this argument in is induction unformalizable?, which is linked to from Berry's Paradox and universal distribution. Did you read it?)