1) RECITING THE BLESSING FOR THE KORBAN PESACH OVER THE KORBAN CHAGIGAH
QUESTION: Rebbi Yishmael in the Mishnah states that the blessing that one recites when he eats the Korban Pesach exempts the Korban Chagigah from its blessing.
(According to the RASHBAM, the respective blessings are "le'Echol ha'Pesach" and "le'Echol ha'Zevach." According to the RAMBAM (Hilchos Chametz u'Matzah 8:7), the blessings are "Al Achilas ha'Pesach" and "Al Achilas ha'Zevach.")
One offers the "Chagigah of the Fourteenth" on the fourteenth of Nisan. He eats it that night, the first night of Pesach, before he eats the Korban Pesach, in order to fulfill the Mitzvah to eat the Korban Pesach "Al ha'Sova," while satiated (70a; see Insights there).
Since the Chagigah is eaten before the Pesach, as the Rambam writes (Hilchos Chametz u'Matzah 8:7), how can the blessing for the Korban Pesach be valid for the Korban Chagigah? A blessing recited upon the fulfillment of a Mitzvah must be recited before the Mitzvah is fulfilled, and not afterwards (7b; see Insights there). (TZELACH)
(a) The OR ZARU'A (Hilchos Keri'as Shema #25) addresses this question. He writes that even though the Korban Pesach is eaten after the Chagigah, the blessing for the Pesach still covers the Chagigah that was eaten earlier. The Or Zaru'a proves from here that a person may recite the blessing for a Mitzvah even after he has performed the Mitzvah (in contrast to the view of the Rambam in Hilchos Ishus 3:23).
(b) The KOL BO (#50) writes that the obligation to eat the Korban Pesach while satiated, "Al ha'Sova," does not require that all of the Pesach be eaten at the end of the meal. Rather, the Mitzvah to eat the Pesach "Al ha'Sova" is fulfilled when one eats merely a k'Zayis of the Pesach when he is full.
(This is evident from the words of the Rambam. When the Rambam describes the order of the Seder, he writes that "one first recites the blessing [for the Korban Chagigah] and eats from the meat of the Chagigah of the Fourteenth, and then he recites the blessing [for the Korban Pesach] and eats from the meat of the Pesach" (Hilchos Chametz u'Matzah 8:7). He then writes that one continues and "eats the Se'udah, and he eats and drinks however much he wants, and at the end he eats from the meat of the Pesach at least a k'Zayis, and he may not taste anything after that" (ibid. 8:9).)
Accordingly, one may eat some of the Korban Pesach first with its blessing, and then eat some of the Chagigah (covered by the blessing for the Pesach). At the end of his meal, he eats a k'Zayis of the Pesach in order to fulfill the Mitzvah of "Al ha'Sova."
The TZELACH comments that according to this approach, the Mishnah refers to a person who ate the Korban Pesach before the Chagigah (which he was not supposed to do l'Chatchilah). He may eat the Chagigah after the Pesach as long as he eats a k'Zayis of the Pesach at the end. (The reason why, l'Chatchilah, one is supposed to eat the Chagigah first is because of the principle of "Tadir v'she'Eino Tadir, Tadir Kodem" -- that which is more frequent takes precedence, and not in order to enable the Pesach to be eaten "Al ha'Sova.")
(c) The CHESHEK SHLOMO and HAGAHOS MAHARSHAM understand the Mishnah in an entirely different way. The Mishnah does not mean that when one eats the Pesach first with a blessing and then he eats the Chagigah, his blessing for the Pesach is valid for the Chagigah. Rather, the Mishnah refers to a case in which a person intended to eat the Chagigah first but accidentally recited the wrong blessing. Instead of the blessing for the Chagigah, he said the blessing for the Pesach. The Mishnah teaches that his blessing is valid, because the blessing for the Pesach is valid for the Chagigah. (This explanation is consistent with the text of our edition of the Mishnah, but not with the text of Rashbam's edition of the Mishnah.)