The Laws of The Beginning of the Month (9) Reciting the Hallel

1. One who recites the Hallel in the wrong order, which means that he said the later verse before the earlier verses of the same paragraph, has not fulfilled his obligation. Some say that even if he changed the order of the paragraphs and said the later one first that he has not fulfilled his obligation. (Shulchan Aruch 422)

2. Therefore, if he erred and omitted a verse, and remembered only after he had already continued further, he should not say only that verse when he remembered, but instead he should return to that verse and continue from there according to the normal order, and repeat even those verses which he had already said. (ibid)