All Muslims accept to some degree that the Prophets of Allah (s.w.t) are Infallible and that these individuals are divinely protected by Allah (s.w.t) from committing errors.
Shi’a Muslims however believe that Infallibility encompasses a much broader scope which they argue is rationally necessary as a result of Allah’s (s.w.t) justice.
If it was the case that Prophets and messengers were fallible and susceptible to the flaws and weaknesses which others experience, this would allow the people to argue against accepting the message of the Prophet (s.a.w).
Shi’a likewise accept that the Infallibility which encompasses the mission of the Prophets is not merely restricted to the period during which the Prophet is declared a Prophet. It also encompasses the entire duration of the Prophets’ lives, based upon the idea that the Prophets should already be recognised as being the most trustworthy in society prior to the commencement of their prophethood.
The doctrines of the Shi’a in regard to Infallibility are primarily based upon scriptural (Qur’an and authentic narrations), as well as rational conclusions which would necessitate that the Prophets and messengers (and Imams) are exemplary role models for mankind to follow.
By necessity they would be unable to violate and transgress against the laws which they are delivering to mankind.