What should one do who made a mistake in the prostration of forgetfulness (sujud al-sahw), such as forgetting to say "Rabbighfir li" (My Lord, forgive me) between the two prostrations, or whose ablution was invalidated during it?

First: It is prescribed in the prostration of forgetfulness (sujood as-sahw) to say in it, and between the two prostrations, what is said in the prostration of the prayer. He is not obliged to perform another prostration of forgetfulness if he forgets something from that, because that would lead to an endless sequence.

Second: If a person's ablution (wudu) is invalidated during the prostration of forgetfulness, there is a detailed explanation:

If the prostration of forgetfulness was before the tasleem (salutation of peace), the prayer becomes invalid due to the invalidation of ablution before performing the pillar of tasleem. If the prostration of forgetfulness was after the tasleem, and the worshipper invalidated his ablution during it or before it, his prayer is valid and there is nothing upon him; because the prostration compensates for the prayer, and the prayer is not invalidated by its omission.