What is the ruling regarding someone who completed their prayer before the Imam prostrated for forgetfulness (sujud al-sahw), and someone who completed it after the Imam prostrated for forgetfulness, in the case that they were latecomers (masbuq)? And what are the consequences for the one who erred?

Scholars differ concerning the ruling on whether a latecomer (masbuq) should follow the Imam in the prostration of forgetfulness (sujud al-sahw) after the salutation (taslim). The Hanafis and Hanbalis are of the opinion that he should follow, based on the generality of the hadith: "And when he prostrates, then prostrate," and because the Imam has not yet finished his prayer. The Malikis and Shafi'is, on the other hand, are of the opinion that he should not follow. Rather, the latecomer should stand to complete his prayer, and then perform the prostration of forgetfulness, because the following (iqtida') ceases with the Imam's salutation, and the aforementioned hadith specifically applies to following during the main part of the prayer. Both opinions are considered valid schools of thought.