What is the ruling when someone replies to the Imam in prayer with an incorrect response, and the Imam then follows him unknowingly?

If the error mentioned is in other than Al-Fatiha, then the prayer is valid for both the Imam and the follower, and there is no sin upon them if the error was due to forgetfulness or ignorance; because the Prophet (peace and blessings be upon him) said: "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are forced to do." However, if the error is in Al-Fatiha and it results in a change of meaning or the omission of some letters, then the prayer is invalid if the error is not corrected. And the follower should not prompt his Imam unless he has memorized correctly what he intends to correct the error with.