Answered by Shaykh Amjad Rasheed
You mentioned that scholars have clearly stated that prayer is valid behind an imam who is excused (on account of his ignorance, for example) if he doesn’t pronounce the definite article in the closing salams, as opposed to one who makes mistakes in the fatiha. Are there other invalidators [m: of prayer] that are excusable if the imam does them out of ignorance such that it is valid for the follower to follow him?
Ignorance is not an excuse unless:
1. one recently accepted Islam, or
2. one lives far from scholars.
As for ignorance that is due to other than the above, it is not excusable with the exception of a few points that are considered too inconspicuous for laypeople, such as if the imam leaves the first tashahhud and stands for the third rakah and then returns to the tashahhud. The original ruling in this case is that the prayer is invalid because he engaged in an integral and then abandoned it for a sunna. However, [m: the scholars] have said that an ignorant person is unconditionally excused in this regard because it is one of the points that is too inconspicuous for laypeople (Hashiya Al-Bajuri, 1.193). So, if the imam does this, the follower completes his prayer behind him.
Amjad Rasheed
Amman, Jordan
(Translated by Moustafa Elqabbany)