What are the circumstances in which the prayer of the congregants is invalidated?

The prayer of the ma'mum (follower) is invalidated if he intentionally follows the imam (leader) in adding a pillar or a rak'ah (unit of prayer), or if he follows him in his forgetfulness concerning the obligatory acts of prayer, except for the recitation of Surah Al-Fatihah according to the Shafi'i school. It is also invalidated if he precedes the imam in the takbirat al-ihram (opening takbir) or the salam (salutation of peace). Furthermore, his prayer is void if he prays behind an imam who is in a state of ritual impurity (hadath), or who is ritually unclean (mutanajjis), or a woman, or an khuntha mushkil (hermaphrodite with ambiguous gender), or a disbeliever. His prayer is also invalidated by following another ma'mum, or by lagging behind his imam by more than two pillars without a valid excuse.