The narrative principle “One who attains the truth has two rewards and the one mistaken has one reward” defined as "the theory of excuse" in this study, is a summary of a tradition on the authority of Amr b. ‘As and Abu Horayreh in the Sunni hadith books which has been mostly referred to in the field of ijtehad (practicing religious jurisprudence), proving the Mokhta`eh principle. This principle has been considered as acceptable by the Shiites. In the present paper, in order to define the reasons for Shiite scholars’ different approach to this principle, from rejection to acceptance and fame, its status among them from the time of Imams (PBUT) up to the contemporary era, has been studied. Meanwhile before dealing with the verdicts of the Shia in this regard, the authenticity of the narration according to Ahl-al Sunnah has been proved primarily, and then their utilization of this principle has been elaborated in order to illustrate the scholastic environment of Islamic society in different centuries. The studies show that after the complete separation of the concept of ijtehad and analogy among Islamic Ummah, the excuse of the mistaken (ma’zuriat mokhti) was formally accepted in seventh century in Shia by Mohaghegh Helli and intellectual evidences were elaborated for it, which was completed and enforced by jurisprudents of the next generations. Then in 13th century, this actually Sunni principle was accepted, due to its coordination with the Shiite principles based on the Book, Sunnah and intellect.