What is the ruling on incomplete rows of worshippers in the Sacred Mosque (Masjid al-Haram) and the Prophet's Mosque (Masjid an-Nabawi)?

There is no harm in praying in a row that is incomplete from the right or the left, but the Sunnah is to complete the rows, the first one then the next, based on the Prophet's (peace be upon him) saying: "Complete the rows, for I see you behind my back." The worshipper should strive to pray in connected rows as much as possible.