What should one do if, having been distracted during congregational prayer, he was late in rising from bowing (ruku'), thereby missing one prostration (sajdah) but catching the second one of the same rak'ah?

As long as the man caught up with the second prostration with the Imam, and his delay was due to forgetfulness, then his prayer is valid and nothing is required of him. If the congregant lags behind the Imam by one or two pillars due to an excuse, he should perform what his Imam preceded him in and catch up with him if possible. If he catches up, then nothing is incumbent upon him. Otherwise, that particular rak'ah becomes void, and he should make it up after the Imam's salutation.