When the Pesah Seder is held on Mosa’eh Shabbat, a special Kiddush is recited, one which incorporates Habdala. The sequence of this Kiddush is known by the acrostic "Yaknehaz," which stands for:
1) Yayin – the Beracha over the wine ("Boreh Peri Ha’gefen");
2) Kiddush – the regular Beracha of Kiddush ("Asher Bahar Banu");
3) Ner – the Beracha over the candle ("Boreh Me’oreh Ha’esh");
4) Habdala – the Beracha of "Ha’mabdil," concluding with "Ha’mabdil Ben Kodesh Le’kodesh";
5) Zeman – the Beracha of "She’hehiyanu."
Maran (Rav Yosef Karo, author of the Shulhan Aruch), in his Bet Yosef (Orah Haim 473), tells that one of the Rishonim, the Orhot Haim, once mistakenly recited the standard Kiddush on the night of the Seder, forgetting that it was Mosa’eh Shabbat, and so he did not recite Habdala. He did not realize his mistake until the middle of the Maggid section. The Bet Yosef rules that in such a case, one should complete Maggid, through the recitation of the Beracha of "Ga’al Yisrael," and then recite Habdala. He should not interrupt Maggid to recite Habdala, and should instead recite it after "Ga’al Yisrael."
The Bet Yosef does not clarify, however, how precisely the Habdala is recited, whether it is recited over the same cup of wine as the cup over which one recites "Ga’al Yisrael." Hacham Ovadia Yosef, in his Teshubot Hazon Ovadia, writes that there are two options. One possibility is to use two cups, which might, at first glance, be preferable, in order to avoid the Halacha of "En Osin Misvot Habilot Habilot" – we do not combine Misvot into a single "bundle." Seemingly, using a single cup of wine for two distinct Misvot – the second of the four cups at the Seder, and Habdala – violates this rule. If so, Hacham Ovadia writes, then one should recite "Ga’al Yisrael" over one cup, put it down, and then pick up a second cup for Habdala, over which he recites the Beracha over the candle and "Ha’mabdil" (concluding "Ha’madil Ben Kodesh Le’kodesh"). He does not have to recite "Boreh Peri Ha’gefen," as this Beracha had been recited at Kiddush. At this point he drinks a Rebi’it from the Habdala cup, and then drinks a second Rebi’it from the other cup.
However, Hacham Ovadia rules that this is not necessary, as it is permissible to recite "Ga’al Yisrael" and Habdala over the same cup of wine. The Rambam (Rav Moshe Maimonides, Spain-Egypt, 1135-1204), in Hilchot Shabbat (29), rules that the principle of "En Osin Misvot Habilot Habilot" applies only when one seeks to combine two Torah obligations together. In the case under discussion, both the requirement of reciting Habdala over a cup of wine and the obligation of the four cups at the Seder were enacted by the Sages, and do not constitute Torah obligations. As such, the rule of "En Osin Misvot Habilot Habilot" does not apply. Therefore, one may simply recite "Ga’al Yisrael" and then Habdala over the same cup of wine, and then drink.
Hacham Ovadia further notes that one should recite "Boreh Me’oreh Ha’esh" over a flame as soon as he realizes his mistake, during Maggid, rather than waiting until after Maggid to recite this Beracha with Habdala.
If one remembered his mistake before eating Karpas, then he should recite Habdala at that point. Since one may not eat on Mosa’eh Shabbat before reciting Habdala, one who neglected to recite Habdala with Kiddush and remembers afterward should recite Habdala before eating the Karpas. Even though one will then be drinking an additional cup of wine, this is what he must do in order to recite Habdala before eating.
The Hafetz Haim (Rav Yisrael Meir Kagan of Radin, 1839-1933), in Be’ur Halacha, rules that if one remembered his mistake after he poured the second cup of wine, but before he began reading Maggid, he should wait until after "Ga’al Yisrael" before reciting Habdala. In his view, pouring the wine before Maggid effectively begins Maggid, and thus one may not recite Habdala from that point until after Maggid. Hacham Ovadia, however, disagrees, and maintains that as long as one had not yet begun reading Maggid, he may recite Habdala and does not have to wait until after Maggid.
If one realized his mistake during the meal, he of course may recite Habdala right there and then. If he realized his mistake during the recitation of Birkat Ha’mazon, then he recites Habdala after Birkat Ha’mazon. If he realized during Hallel, then he recites Habdala after Hallel.
Summary: When the Seder is held on Mosa’eh Shabbat, one must recite Habdala in conjunction with Kiddush, following the proper sequence. If one realized after Kiddush, but before Karpas, that he had not recited Habdala at Kiddush, the he must recite Habdala over a cup of wine at that point. If he realized his mistake during Maggid, he should recite the Beracha over the candle at that point, and recite the Beracha of Habdala at the end of Maggid, right after the Beracha of "Ga’al Yisrael," over the same cup of wine over which he recites "Ga’al Yisrael." If he realizes during the meal, he should recite Habdala at that point, and if he realizes during Hallel, then he should recite Habdala after Hallel.
