The most distinguished merit of the Ahl al-Bayt (a) is their infallibility which can clearly be understood from the verse of Tathir, since in this verse, Ahl al-Bayt (a) are referred to as those whom God desired to repel all impurity from. The word "إنما" ["Innama"] in this verse [Tathir] and the hadiths about the cause of its revelation shows that this [being free from all impurities] is among the merits of the Ahl al-Bayt (a) and it is special to them.
Hadith of Thaqalayn is also among Mutawatir [frequently narrated] hadiths which has no doubt in its reference and it proves the infallibility of the Ahl al-Bayt (a) of the Prophet (s) (in its very special usage), because in this hadith, Ahl al-Bayt (a) have come as Thiql Asghar [the smaller weight] together with the Qur'an which is Thiql Akbar [the greater weight] and they have been considered as the two legacies of the Prophet (s) which would never be separate from each other and if Muslims adhere to them, they will never be misguided.
Indeed the noble Qur'an is the word of God and no error is in it, "falsehood cannot approach it, from before it nor from behind it," (41:42) Therefore, the Ahl al-Bayt (a) of the Prophet (s) who are with the Qur'an, and adhering to them prevents human being to go astray.
According to some Sunni scholars, there is no doubt in the moral and practical infallibility of the Ahl al-Bayt (a) who include lady Fatima (a) and the twelve Imams (a) of Shi'a and no one would doubt in it unless an ignorant person who rejects Islam. What is disagreed is their practical infallibility; however because the hadith of Thaqalayn regards adherence to the Ahl al-Bayt (a) in religion keeping one safe from ignorance and going astray, their practical infallibility can easily be understood.