What should the worshiper do if the Imam commits an act that invalidates the prayer, and this act is a matter of scholarly disagreement, or if there is a consensus that it invalidates the prayer?

If the Imam believes that an act does not invalidate his prayer, but the congregant (Ma'mum) believes it does, then the prayer of both the Imam and the congregant is valid according to the majority of the Salaf (early Muslims). This is the madhhab (school of thought) of Malik, and one of the two opinions of Al-Shafi'i, Ahmad, and Abu Hanifa, who cite the hadith: "They pray for you; if they are right, it is for you and for them, and if they are wrong, it is for you and against them." The Imam's error does not extend to the congregant.

However, if the congregant is certain that the Imam performed an act that is impermissible in his (the congregant's) view, there is a dispute, but the correct opinion is that the congregant's prayer remains valid.

If it is possible for the congregant to perform what his Imam omitted, then he should do so.

But if the act committed by the Imam invalidates the prayer without any dispute, such as if he prays in a state of ritual impurity (hadath) while knowing it, and the congregant also knows it, then everyone's prayer is invalid.