What is the ruling regarding a congregant who caught the opening Takbir (Takbirat al-Ihram), then the Imam forgot a raka'ah: should the congregant stand up to complete it, or should he make the Taslim (concluding salutation) with the Imam? And what is the ruling regarding a latecomer who missed one or two rak'ahs before joining the prayer with the Imam?

If the Imam forgets a ruk'ah and leaves it out, it is obligatory for the congregants to alert him by saying "Subhan Allah" (Tasbeeh). If he does not respond, they are not permitted to follow him. Instead, they should stand up to complete their prayer individually. As for the latecomer (masbuq) who stood up to complete his prayer after the Imam's salutation, and then the Imam returned to complete his prayer, the latecomer has a choice: either to rejoin the Imam or to complete his prayer individually.