What should worshippers do if the imam forgets a pillar or an obligation of prayer, whether intentionally, out of forgetfulness, or because he believes he is correct?

If the Imam intentionally omits a pillar (rukn) or an obligatory act (wajib) of the prayer, his prayer becomes invalid, and the prayer of the congregants (ma'mumoon) also becomes invalid according to many scholars. However, if the Imam forgets a pillar or an obligatory act, there are detailed rulings mentioned in previous fatwas.