Sheikh `Abd al-Rahmân al-`Ajlân, lecturer at the Grand Mosque in Mecca
If the imam makes a mistake in reading Sûrah al-Fâtihah to the extent that he changes the meaning of what he is saying, then his prayer and the prayer of the people who follow him will be invalid.
Let us take the following as an example:
In the last verse of Sûrah al-Fâtihah it reads: “Sirât alladhînah an`amta `alayhim”. If the imam reads it as “Sirât alladhînah an`amtu `alayhim”, he will be changing the meaning from “…the way of those upon whom You have bestowed your grace” to “…the way of those upon whom I have bestowed my grace.”
In this case, the prayer will be invalid.
However, if the mistakes made by the imam in recitation concerns only matters of tajwîd (proper recitation of the text) without any alteration in meaning taking place, then the prayer will be valid.
And Allah knows best.
Source: Islam Today