On Shabbat, one must add the paragraph of "Ritze Ve'hachalitzenu" whenever he recites Birkat Ha'mazon. If one forgot to add this paragraph in Birkat Ha'mazon after either of the first two meals of Shabbat, then the following rules apply:

1) If a person realized his mistake after he recited the Beracha of "Boneh Yerushalayim," but before he began the next Beracha, then he recites a special Beracha in lieu of "Ritze." The text of that Beracha is as follows: "Baruch Ata Hashem Elokenu Melech Ha'olam Asher Natan Shabbatot Le'menucha Le'amo Yisrael Be'ahava Le'ot U'berit; Baruch Ata Hashem Mekadesh Ha'Shabbat." If a person finds himself in this situation and he does not have the text in front of him (and he does not know the text by heart), then, as Chacham Ovadia Yosef writes in Halichot Olam (vol. 2, page 75), he should go back to the paragraph of "Rachem" and continue from there as usual, adding "Ritze."

2) If the person realized his mistake only after he recited the words "Baruch Ata Hashem Elokenu Melech Ha'olam" of the subsequent Beracha of Birkat Ha'mazon, then according to Chacham Ovadia Yosef he may then continue with the aforementioned Beracha, "Asher Natan Shabbatot…" Even though when he recited the words "Baruch Ata Hashem Elokenu Melech Ha'olam" he had in mind to continue with the next Beracha of Birkat Ha'mazon, he is nevertheless permitted to continue instead with the Beracha of "Asher Natan Shabbatot."

3) If he realized his mistake only after he recited the word "La'ad" in the fourth Beracha of Birkat Ha'mazon, the he must repeat the entire Birkat Ha'mazon from the beginning.

These rules apply only to the Birkat Ha'mazon recitation after the first two meals of Shabbat. Separate rules apply to a case where one forgot to recite "Ritze" in Birkat Ha'mazon after Se'uda Shelishit (the third Shabbat meal).