(Mishnah - R. Eliezer): If blood that requires four Matanos (being thrown against walls of the Mizbe'ach) was mixed with blood that requires one, we do four Matanos;

R. Yehoshua says, we do one Matanah.

R. Eliezer: You transgress Bal Tigra (detracting from a Mitzvah)!

R. Yehoshua: You transgress Bal Tosif (adding to a Mitzvah)!

Rosh Hashanah 28b - Abaye (to Rava): (If you hold that one is Yotzei without intent,) you should say that one who sleeps in a Sukah on Shemini Atzeres is lashed for Bal Tosif!

(Rava): One is Yotzei a Mitzvah without intent. One transgresses Bal Tosif only with intent (to add).

Question: R. Yehoshua holds that one transgresses without intent!

Correction (Rava): One transgresses Bal Tosif at the time of the Mitzvah even without intent. One transgresses not at the time of the Mitzvah only with intent.

Sukah 47a: The Halachah is, we sit in the Sukah on Shemini Atzeres, but we do not bless (Leshev ba'Sukah).

Rif and Rosh (Sukah 4:5): The Halachah is that we sit in the Sukah, but do not bless. Since it is Shemini Atzeres, it is a contradiction to bless (this would show that it is the seventh day). We are stringent to sit in the Sukah and conduct like Yom Tov.

Rambam (Hilchos Sukah 6:11): On Hoshanah Rabah, one removes his Kelim from the Sukah in the afternoon. If he has no place to move his Kelim to, he opens a place (in the Sechach) four by four Tefachim.

Ra'avad: Perhaps he means that on other days,if one needs to eat in the Sukah, he need not breach it, but on Shemini Atzeres, one could thnk that he does so l'Shem Mitzvah, so he must open it.

Rambam (13): Nowadays that we keep two days of Yom Tov, we sit in the Sukah eight days. On the eighth day (Shemini Atzeres) we sit in it, but we do not bless Leshev ba'Sukah.

Tosfos (Rosh Hashanah 16b DH v'Tok'im): Why may we blow extra Teki'os? This is Bal Tosif. One transgresses even without intent, for the Mitzvah applies the entire day, if another Tzibur that did not yet hear them will present itself! We must say that repeating a Mitzvah is not Bal Tosif.

Rashba (Rosh Hashanah 16a DH Lamah): Bal Tosif is what a person deciees on his own. An enactment of Chachamim is not Bal Tosif. This is why we sleep in the Sukah on Shemini Atzeres. We are obligated to follow their words!

Shulchan Aruch (OC 666:1): If one has no place to move his Kelim and he wants to eat in the Sukah on Shemini Atzeres, he must open a place (in the Sechach) four by four Tefachim to make it clear that he does not intend for the Mitzvah, lest he look like one who adds to the Mitzvah.

Taz (1 and Magen Avraham 2): He does not truly add to the Mitzvah, for after the time one transgressed Bal Tosif only with intent to add.

Gra (1): In Eruvin, Rashi says that we asked (will you say that one who sleeps in a Sukah on Shemini Atzeres is lashed) because this is our practice. The Rashba (Rosh Hashanah one to another DH Lamah) says that Bal Tosif does not apply to any enactment of Chachamim. We blow extra Teki'os mid'Rabanan, we oberve Yom Tov Sheni, and we detract from Mitzvos (we do not blow the Shofar when Rosh Hashanah is on Shabbos). The Rashba and Ritva say that we sleep in the Sukah on Shemini Atzeres, unlike the Mordechai.

Mishnah Berurah (4): Even though three Tefachim of air disqualifies a Sukah, we require a large amount so it will be evident.

Rema: If he wants to eat in the Sukah after the Chag, he need not open anything. It looks like adding only on Shemini Atzeres.

Beis Yosef (Sof Siman 651 DH u'Mah): The Tur says that one who adds another species to the four Minim transgresses Bal Tosif only if he takes it l'Shem Mitzvah. In Rosh Hashanah, we learned that at the time of the Mitzvah, one transgresses even without intent!

Magen Avraham (27): R. Yerucham said that this (at the time of the Mitzvah, one transgresses even without intent) is according to Rava, who holds that Mitzvah do not require intent. We rule (OC 60:4) that Mitzvos require intent, so one transgresses only with intent. The Magid Mishneh and Ran hold that one does not really transgress, for the extra species is separate, but one should not do so, for he looks like one who adds. Perhaps this is the intent of the Tur and Beis Yosef.

Shulchan Aruch (OC 668:1): In Chutz la'Aretz we eat in the Sukah at night and during the day, but we do not bless because it is a Safek.

Beis Yosef (DH Kosav): The Mordechai says that since we do not bless l'Chatchilah we did the last seven days, there is a Heker, and it is not like adding to the Mitzvah. One may not sleep, for this looks like adding. The Poskim did not distinguish (they permit even sleeping).

Magen Avraham (2): We eat in the Sukah because there is a Heker that we do not add to the Mitzvah, i.e. we do not bless. We do not sleep in the Sukah because there is no Heker (we never bless on sleeping)! This is why we leave the Sukah right after eating (there is no Heker when we stay in the Sukah without eating).

Mishnah Berurah (6): Many require sleeping in the Sukah on Shemini Atzeres. The custom is to be lenient. Some say that those who forbid sleeping also forbid eating something (e.g. fruits) on which we never bless Leshev ba'Sukah, for there is no Heker. Others permit, and this is more reasonable.

Kaf ha'Chayim (6): Benei Eretz Yisrael in Chutz la'Aretz should eat in the Sukah on Shemini Atzeres, but they should explicitly say (inaudibly to others) that it is not l'Shem Mitzvah. They should find an excuse to sleep in the house, and in private eat something in the house. They should not be conspicuous, lest Benei Chutz la'Aretz learn from them to be lenient.

Beis Yosef (DH u'Mah she'Chosav v'Yesh): Some sit in the Sukah on the day of Shemini Atzeres, but not at night. This is because Yeshivas Sukah contradicts calling the day Shemini Atzeres in Kidush.

Rebuttal (Magen Avraham 2): Also during the day we call the day Shemini Atzeres in Birkas ha'Mazon! Rather, these people are concerned for Bal Tosif (adding to a Mitzvah). After ceasing to eat in the Sukah for an evening, we are not concerned for Bal Tosif. However, all the Poskim require eating all meals in the Sukah on Shemini Atzeres, even Bein ha'Shemashos at the end of the day. This is why Noyei Sukah are forbidden on day nine.

Yom Tov Sheni k'Hilchaso (2:7 and footnotes): Some say that someone from Chutz la'Aretz visiting Eretz Yisrael must sit in a Sukah on Shemini Atzeres without a Berachah (Be'er Moshe, ha'Ga'on S. ha'Levi Vozner, ha'Ga'on R. C.P. Sheinberg). Ha'Ga'on R. Y.S. Elyashiv says that one should have pots in the Sukah at the time, for then even a Ben Eretz Yisrael may sit inside. Others exempt (Lu'ach Eretz Yisrael). Ha'Ga'on R. S.Z. Auerbach exempts because even in Chutz la'Aretz there are different customs.