Is the prostration of forgetfulness (sujud al-sahw) obligatory upon the follower (ma'mum) if he makes a mistake in following the Imam in congregational prayer (such as standing or sitting in the wrong place), and should he join the prayer with the Imam during the final tashahhud and then make up what he missed, or should he discontinue his prayer if the Imam makes the taslim directly thereafter?

If you make a mistake behind the Imam, such as standing when the Imam is sitting or vice versa, you are not required to perform the prostration of forgetfulness (sujud as-sahw), because the Imam bears the responsibility for the mistakes of the congregant. There is no forgetfulness required of the congregant, as Ibn al-Mundhir said: "They have unanimously agreed that there is no prostration [of forgetfulness] required of one who makes a mistake behind the Imam." If a latecomer (masbuq) says the takbir for ihram and the Imam says the taslim before the latecomer sits, the latecomer should not sit or interrupt his prayer. Instead, he should continue with it, recite the opening supplication (dua al-istiftah) while standing, and then proceed to recite Al-Fatiha.