What should the congregant do if the Imam inadvertently omits a pillar of prayer, and the Imam does not notice the congregants' alert or no one alerts him?

If the Imam forgets a pillar of the prayer, such as prostration, the congregants should alert him by saying "Subhanallah" (Glory be to Allah). If he does not understand by "Subhanallah", they should speak to him as much as needed.

If he does not comply, they should perform the pillar that the Imam omitted for themselves and not follow him.

Alerting the Imam to his oversight may be obligatory in some cases, especially if not alerting him would lead to the invalidation of the prayer.