What is the original dogmatic fault Shi'is? What are the differences between the Shi'is and Sunnis?
Allow me not to comment here about the false and truth and simply illustrate the difference between the Shia Muslims and the mainstream Muslims. The mainstream Muslims consider the only divine source of Islam, existing after the Prophet (sws) to be the Qur'an and Sunnah. However according to the Shia Muslims (the majority of them who are known as Imami Shia), beside the Qur'an there are twelve individuals from the generation of the Prophet (sws) who came one after the other as the divinely appointed Imams of the Ummah after the Prophet (sws). According to the Shia Muslims these individuals are infallible and where needed can have access to the hidden knowledge. Accordingly they believe that only the guidance given by these Imams is the true guidance and any one who follows other understandings of Islam is misguided or not fully guided. They therefore consider the Khulafa of Muslims (from Abubakr to the last one, except Ali – ra – who they consider to be the first of these divinely guided Imams) to be transgressors of the right of these Imams.
According to the Shia Muslims, the last Imam is the promised Mahdi who was born in the third century and then went to occultation a few years later to remain safe till people are ready for his return. They believe that he is miraculously still alive and will return before the end of the world.