Arthur does not allow Ariadne to touch his totem, implying that doing so would give its owner reason to mistrust the results or would somehow allow the totem to be "contaminated" and therefore capable of providing inaccurate readings. Cobb tells Ariadne that his totem used to belong to Mal, which would seem to violate the rule of using a totem that someone else has used or touched. Even worse, Mal killed herself because she was absolutely convinced that she was still in a dream and not in reality. That means her totem (that Cobb now uses) told Mal one of two things. It either, correctly or incorrectly, revealed that she was in reality, but she felt she had an extremely good reason not to trust it or it demonstrated, correctly or incorrectly, that she was still in a dream and she chose implicitly to believe the result. Either way, it would seem foolhardy for Cobb to rely on a totem with such a dubious history.