What is the Ruling on Following an Imam Who Made a Mistake Due to Forgetfulness?

However, I want to bring the attention of some people who find following the Imam strange when they know that he has made a mistake. So, who among us doesn't follow the Imam, if he forgets the middle Tashahud and stands for the third rakaah? Everyone who's behind him knows that he has made a mistake when he stood to the third rakaah and left off the Tashahud. So, if he left off the Tashahud intentionally, then it is a sin. This is not expected to come from an Imam, and it didn't happen in the past or present. However, if it was by mistake and forgetfulness, then following him is a must as clearly reported by the Prophet ﷺ.