What is the ruling when one mistakenly corrects the Imaam's recitation during prayer and then the latter follows him in this mistake?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If the mistake in recitation is not in Al-Faatihah, then the prayer of both the Imaam (i.e. the one who leads others in prayer) and the one following him is valid. As long as this is done out of mistake, forgetfulness or ignorance, they incur no sins at all. The Prophet, sallallaahu ‘alayhi wa sallam, said: "Allaah has forgiven my Ummah (i.e. nation) for mistakes and forgetfulness, and what they are forced to do.'' [Ibn Maajah and others; Al-Albani: authentic]

However, if the mistake occurred in Al-Faatihah and resulted in a changed meaning or the omission of some letters; then the prayer is invalid, unless the mistake is corrected.

One must not correct the Imaam's mistake, unless he is absolutely certain of his memorization of the part he wants to correct, so as not to confuse the Imaam.

Allaah Knows best.