What is the ruling on the prayer of one who prostrated for forgetfulness before the Taslim, then doubted the number of prostrations and added another prostration, then performed two prostrations of forgetfulness again after the Taslim? And what is the ruling on the prayer of a latecomer who delays slightly after the Imam's Taslim to complete the Tashahhud before standing up?

If one is certain of having made a mistake (in prayer) but doubts whether he performed the prostration of forgetfulness (sujud al-sahw) for it or not, then he should perform it, because the default is that it was not performed. And if he doubts whether he performed one prostration or two, he should perform another. He should not prostrate a second time for forgetfulness in the prostration of forgetfulness itself, or for doubting it, nor should he prostrate after the salutation (tashahhud).

As for the latecomer (masbuq) remaining seated after the Imam's salutation: if it was the place for his first tashahhud, it is permissible and his sitting may be prolonged, and his prayer is not invalidated. But if it was not the place for his tashahhud, and he sat intentionally and knowingly, his prayer is invalidated. If he sat forgetfully, his prayer is not invalidated and he should perform the prostration of forgetfulness.