The Shi'a school of thought maintains the belief that all the prophets of Allah, from Adam to Muhammad, as well as the twelve successors (imams) of the Prophet Muhammad, and his daughter, Lady Fatima al-Zahra were infallible throughout their entire lives and never committed any type of sin that would earn the displeasure of Allah. The clearest way to see this point is to consider that these people were the examples sent for humanity to follow, and thus, if they committed errors then people would be obliged to follow their errors, thereby rendering the prophets and messengers unreliable.
Infallibility means protection. In Islamic terminology it means the spiritual grace of Allah enabling a person to abstain from sins by his/her own free will. The power of infallibility or without sin does not make a person incapable of committing sins, rather he/she refrains from sins and mistakes by his/her own power and will.
Infallibility is essential for the prophets and messengers because their job is not only to convey the divine scriptures of Allah but also to lead and guide humanity towards the right path. Therefore, they must be role models and perfect examples for all of mankind. Both the Noble Quran and conventional wisdom illustrate this point; the Noble Quran mentions infallibility thirteen times. Allah says to Satan, “Certainly you shall have no authority over My servants except those who follow you and go astray.” Satan thus replied to Allah, “By Your might, I will surely mislead all of them, except Your chosen servants among them (the messengers and the imams).”
There are some verses in the Noble Quran which might imply that some of the prophets (such as Adam, Musa (Moses), or Yunus (Jonah)) committed sins. As for Prophet Adam, he did not disobey the obligatory commands of Allah; the command that he did not honor was a recommended one, not a mandatory one and so—according to Islamic terminology—he did not commit a sin. When speaking about the “disobedience” of Prophet Adam, the Noble Quran does not mean disobedience in the literal term; it means that it was not expected from a person like Prophet Adam, who was a leader for humanity, not to adhere to Allah's advisory commands. Therefore, such an act is labeled allegorically as a sin in the Noble Quran. “And indeed We made a covenant with Adam before, but he forgot, and We found on his part no firm will-power ('azm).” His guilt was that he did not demonstrate will-power, not that he violated Allah's rules because the commands were advisory and not obligatory. As a result of his behavior, he was to lose the privilege granted to him, “Verily, you have a promise from Us that you will never be hungry therein, nor naked, and you will not suffer from thirst therein, nor suffer from the sun's heat.”
As for Prophet Musa, the Noble Quran says about him saying the following, “And they have a charge of crime against me, and I fear that they will kill me.” This charge came about when he pushed a man and inadvertently killed him. At that time, Prophet Musa was defending one of his tribesmen, and when he pushed the man from the people of Pharaoh it happened that the man was so weak that he fell to the ground and died. Prophet Musa did not intend to kill him, and thus he fled the scene because he did not want to fall captive to Pharaoh and his army, which was searching for him. When Prophet Musa speaks of them having a “charge of crime” against him, he is reiterating the accusations of the Pharaoh's people, not necessarily believing that those accusations are true.
The case of Prophet Yunus (Jonah) is similar. The Quran says, “And remember, when he went off in anger, and he imagined that We would not confine him. But he cried through the darkness, saying, 'There is no Lord except You. Glory be to You! Surely, I have been one of those who did injustice to their own souls.'” In this case, Prophet Yunus meant that he had been wrong to himself, but wrongdoing one's self is neither a sin nor a mistake. His “wrongdoing to himself” was being impatient with his followers and fleeing from them when they persisted in rejecting his call to worship Allah. They ridiculed him and thus he left them to face their grave destiny.
Most of the verses of the Noble Quran which might imply that the Prophet Muhammad committed a sin have deeper hermeneutic interpretations. Not all of the verses of the Quran are meant to be taken literally; in-fact deeper meaning lies behind many of them. “It is He who has sent down to you the Book. In it are verses which are entirely clear. They are the foundations of the book. And (there are) others that are not entirely clear (i.e. allegorical); so as for those in whose hearts there is a deviation, they follow that which is not entirely clear thereof, seeking dispute (fitna) and seeking to distort the true meaning. But none knows the hidden meanings save Allah and those firmly grounded in knowledge (the Prophet and the Ahul-Bayt).” Furthermore, the character and general respect accorded to the Prophet shows without any doubt that he was not one of the wrongdoers.
Inappropriate narrations are found in some books of hadith regarding violations committed by various prophets of Allah. For example, Imam al-Bukhari narrates:
Umar sought permission from the Messenger of Allah to visit him when some women of Quraysh were busy talking with him and raising their voices above his voice. When Umar sought permission, they stood up and went hurriedly behind the curtain. The Messenger of Allah gave him permission smilingly. Thereupon Umar said, 'O Messenger of Allah, may Allah keep you happy all your life.' Then the Messenger of Allah said, 'I wonder at those women who were with me, and that no sooner did they hear your voice that they immediately wore the hejab.'
Similarly, Imam Muslim narrates about the Noble Prophet as follows:
Abu Bakr came to see me and I had two girls with me from among the girls of the Ansar, and they were singing what the Ansar recited to one another at the Battle of Bu΄ath. They were not however singing girls. Upon (seeing) this, Abu Bakr said, 'What? This wind instrument of Satan (being played) in the house of the Messenger of Allah, and this too on Eid (Muslim holiday) day?' At this, the Messenger of Allah said, 'Abu Bakr, all people have a festival, and this is our festival (so let them play on).' 
It has also been narrated that the Prophet Muhammad was seen standing and urinating in public. Clearly, acts which the first and second caliphs and the laymen alike would consider un-Islamic would not have been done openly by the Prophet of Allah. No Muslim would accept such behavior from the leader of humanity whose example the Noble Quran commands to be followed in all aspects.
In the books of hadith, there are other unreliable narrations which contradict wisdom and common sense.
There are similar narrations also about some of the other prophets of Allah, for example:
The Angel of Death came to Musa and said, 'Respond to (the call of) Allah (i.e. be prepared for death).' Musa gave a blow to the eye of the Angel of Death and knocked it out. The Angel went back to Allah and said, 'You sent me to Your servant who does not want to die, for look he knocked out my eye.' Allah then restored his eye.
If an ordinary Muslim person attacked someone who was doing his duty, then he would be called an abuser, and an offender and charges would be brought against him. Thus, such behavior is completely unbelievable and unacceptable especially if that person is one of the five universal prophets sent to guide, enlighten, and educate people by their fine examples and morality. Why would one, such as Prophet Musa attack the Angel of Death who came to bring him closer to Allah? Narrations like this one are completely not authentic or acceptable. Muslims must open their eyes to such stories in the books of hadith which have no harmony with the teachings of the Noble Quran.
 Noble Quran, 15:42
 Noble Quran, 38:82
 Noble Quran, 20:118-119
 Noble Quran, 26:14
 Noble Quran, 21:87
 Noble Quran, 3:7
 Sahih al-Bukhari, “Book on the Beginning of Creation” Hadith 3051, “Book on Outstanding Traits” Hadith 3407, “Good Manners” Hadith 5621; Sahih Muslim, “Book on the Merits of the Companions” Hadith 4410; Musnad Ahmad ibn Hanbal, Vol. 1, 171, 182, and 187
 Sahih al-Bukhari, “Book on Friday Prayer” Hadith 897; Sahih Muslim, “Book on the ‘Eid Prayers” Hadith 1479; al-Nisa΄i, “Book on the ‘Eid Prayers” Hadith 1575-1577 and 1579; Sunan ibn Majah, “Book on Marriage” Hadith 1888; Musnad Ahmad ibn Hanbal, Part 6, 166, 186, and 247
 Sahih Muslim, Bab al-Hirab wal-Darq Yawm al-‘Eid; Sahih Muslim, “Book of Taharah” Ch. 22; Sahih al-Bukhari, “Book of Wudu” Vol. 1
 Sahih al-Bukhari, “Book on Funerals” Hadith 1253; Sahih Muslim, “Book in the Virtues” Hadith 4374; al-Nisa΄i “Book on Funerals” Hadith 2062; Musnad Ahmad ibn Hanbal, Vol. 2, 269, 315, 351, and 533