What is the ruling concerning my prayer if I made a mistake in the Qiyam prayer (night vigil prayer), and I thought I was in the first rak'ah, so I stood up, then the worshippers alerted me, so I returned to the tashahhud and concluded the prayer without performing the prostration of forgetfulness (sujood al-sahw)? And does a supererogatory prayer (nafilah) entail the prostration of forgetfulness?

Voluntary (nafl) prayers are similar to obligatory (fard) prayers in the rulings concerning the prostration of forgetfulness (sujud al-sahw). Here, the prostration of forgetfulness is a Sunnah and not obligatory. It ought to have been performed, but since it did not happen, there is nothing upon the worshipper and their prayer is valid. Some scholars, particularly the Malikis, hold the view that it is permissible to perform the post-prayer prostration of forgetfulness (sujud al-sahw al-ba'di) even if a long time has passed.