Narrated Abu Huraira:
Allah’s Apostle said, “If the Imam leads the prayer correctly then he and you will receive the rewards, but if he makes a mistake (in the prayer) then you will receive the reward for the prayer and the sin will be his.”
If the Imaam makes a mistake during Salah, you won’t be punished for it. Politely correct the Imaam for his mistake after Salah. He’s a human, he forgets too. But don’t charge off announcing that everybody’s Salah will be void because of the one mistake the Imaam made.