If the Imam makes a mistake and sits for the tashahhud in the third rak'ah, and the worshipper does not realize this until after he has stood up, should he then sit and alert the Imam, or should he complete his standing and then alert the Imam?

If the Imam sits in an improper place, such as sitting for the tashahhud after the third rak'ah in a four-rak'ah prayer, the ma'mum (follower) should not follow him, but rather alert him. And if he does not follow him in his unlawful sitting, then it is even more appropriate that he should not return to sit with him after the Imam has stood up; instead, he should alert him by saying Subhanallah. Sheikh Al-Akhdari Al-Maliki said: "And if your Imam stands up from two rak'ahs, alert him by saying Subhanallah. If his feet leave the ground, then follow him. But if he sits in the first or third rak'ah, then stand up and do not sit with him."