What is the ruling on a worshipper being slightly behind the row?

The Sunnah is to straighten the rows, and it may be obligatory, based on the Prophet's (peace be upon him) statement to someone whose chest he saw protruding: "You must straighten your rows, or Allah will cause disagreement to appear on your faces." This is a stern warning that indicates the impermissibility of abandoning the straightening of rows or its obligation. Al-Bukhari titled a chapter on this: "Chapter: The Sin of One Who Does Not Complete the Rows."