 Errors can never be completely ruled out in case of verification of a proof by humans either, especially if the proof contains natural language and requires deep mathematical insight to uncover the potential hidden assumptions and fallacies involved.A statement that is neither provable nor disprovable from a set of axioms is called undecidable (from those axioms).One example is the parallel postulate, which is neither provable nor refutable from the remaining axioms of Euclidean geometry.