Is it incumbent upon one who mispronounces the Fatiha by replacing "an'amta" (You have bestowed Your grace) with "an'amtu" (I have bestowed grace) to make up for all the prayers he has performed since puberty until his discovery of the error?

If the melodic error (laḥn) alters the meaning, the prayer becomes invalid if done intentionally, and it must be repeated. However, if it was due to forgetfulness or ignorance, there is a difference of opinion among jurists: some consider the prayer valid, while others deem it invalid. This depends on the worshiper's state and their ability to learn or follow someone who does not make such errors.