What is the ruling on the prayer of a Hanafi congregant behind a Shafi'i Imam who intentionally or unintentionally omits the recitation of Al-Fatiha, while the Imam believes it to be a pillar of prayer, or while the congregant doubts whether the Imam believes it to be a pillar?

Prayer is valid behind someone who differs in minor jurisprudential issues. This is a matter upon which the Companions and the Successors reached a consensus. The reasoning behind this is that the differing individual is either correct and receives two rewards, or mistaken and receives the reward for ijtihad (independent reasoning). If the worshipper is certain that the Imam did something that is impermissible according to his (the worshipper's) madhhab, the worshipper's prayer is still valid according to the preponderant opinion of the scholars, based on the hadith: "They pray for you; if they are correct, it is for you and for them, and if they are mistaken, it is for you and against them." However, if the Imam intentionally omits a pillar that the worshipper believes to be obligatory, the Imam's prayer becomes invalid, but the worshipper's prayer does not become invalid, according to the preponderant opinion. An exception to this is the invalidity of prayer behind someone who mispronounces Surah Al-Fatihah in a way that alters its meaning, unless the worshipper also mispronounces it similarly.