How is the non-performance of prostration of forgetfulness (sujud al-sahw) by a worshiper (ma'mum) who adds a pillar (such as an extra bowing or prostration) consistent with the Imam's non-responsibility for deficiencies in pillars? Is the reason for this that prostration of forgetfulness is obligatory (wajib) and not a pillar? Does this ruling include the latecomer (masbuq) worshiper?

The Imam, as a guarantor, bears the forgetfulness of the congregant unless the congregant omits a pillar. So, an unintentional addition of a pillar, or an unintentional addition of an obligatory act (wajib) or a Sunnah, is borne by the Imam, provided that the forgetfulness occurred on the part of the congregant before the Imam’s salutation. This responsibility for the congregant continues even if he thought the Imam had given the salutation and so the congregant also gave the salutation, but then it became clear that his assumption was incorrect. In such a case, he gives the salutation with or after the Imam and does not perform the prostration of forgetfulness (sujood al-sahw) while still following the Imam. If the congregant remembers in his tashahhud that he omitted a pillar after he had followed the Imam, he does not go back to make it up. Instead, he stands up after the Imam’s salutation to perform the missed rak'ah and does not perform the prostration of forgetfulness.

However, if the congregant doubts whether he omitted a pillar, he must perform it and then perform the prostration of forgetfulness. The Imam does not bear this for him, because the doubt occurred after the Imam’s salutation.

As for the intention (niyyah) and the opening takbeer (takbeerat al-ihram), the Imam does not bear these for the congregant because whoever omits them is not considered to be in prayer to begin with. Similarly, forgetfulness that occurs after the Imam’s salutation is not borne by the Imam because the congregation (qudwah) has ended.

An unintentional addition of a pillar does not invalidate the prayer, and the Imam bears it. This is in contrast to omitting a pillar, whether intentionally or unintentionally, as this invalidates the validity of the prayer, and the prostration of forgetfulness does not suffice to compensate for it.