Gemara

(Beraisa): If there were two pieces (Chelev and Shumen, i.e. permitted fat), and Reuven ate one b'Mezid, and then the other b'Shogeg, he is exempt;

Rebbi is Mechayev.

If he ate both b'Mezid, he is totally exempt.

24b (Rabah): One is Makdish Asham Vadai only if he will need it. If he found out that he did not sin, it was Chulin;

Regarding Asham Taluy, since he was worried, he was Makdish it absolutely.

Contradiction (Rabah): The Mishnah contradicts itself regarding Asham Vadai!

If he found out after Shechitah, it is buried. If the blood was thrown, the meat is burned in Beis ha'Sereifah!

Resolution (#1) (Rabah): We are forced to say that different Tana'im taught these two clauses!

Resolution #2 (Rav Ashi): (Letter of the law, in both cases it should be buried. After Zerikah) it is a decree to burn the meat, for it looks like a Pasul Korban (which is burned).

26b (Rava): "Oh Hoda Elav" obligates bringing a Chatas when he finds out (that he really ate Isur).

The Asham Taluy protects him from punishment (until then).

Kesuvos 22b (Beraisa): If two witnesses say that Ploni (Leah's husband) died, and two say that he did not, she may not remarry. If she remarried, she may remain married.

Question: Perhaps Ploni is still alive. One who has Bi'ah with her must bring an Asham Taluy. How can we let them stay married?!

Answer (Rav Sheshes): She married one of the witnesses who say that Ploni died.

Question: She must bring an Asham. (She is unsure.) How can we let her stay married?

Answer: The case is, she is sure.

Nazir 23a (Beraisa): "He did not know, and sinned" refers to one who ate a piece of Safek Chelev, Safek Shumen (permitted fat). The verse says "he will bear his sin";

Isi ben Yehudah says, there were two pieces of meat in front of him, one of Chelev, and one of Shumen. (Note: Orach Mishor and others fix the text to say that Isi expounds a different verse regarding Asham Taluy "... v'Nislach Lo.")

Had the Torah taught only the case of one piece, one might have thought that only then he gets atonement, for there was no Vadai Isur, but not when there were two pieces, one of which was Vadai Chelev.

Rishonim

Rambam (Hilchos Shegagos 8:4): If there were two pieces, Chelev and Shumen, in front of Reuven, and he ate one b'Mezid, and then the other b'Shogeg, or the first b'Mezid and the latter b'Shogeg, he brings Asham Taluy, since there was an Isur Kavu'a. If he ate both b'Mezid, he is exempt from a Korban.

Kesef Mishneh: The Rambam rules like Rebbi, who obligates even when there was only one piece.

Rambam (Hilchos Pesulei ha'Mukdashim 4:19): If one brought Asham Taluy and he found out that Vadai he did not sin, or Vadai he sinned, if he found out before Shechitah, it grazes until it gets a Mum. It is redeemed, and the money goes to Nedavah. Because a person worries about his sin, and he separated it due to Safek, he resolved to be Makdish it absolutely.

Rashi (Nazir 23a DH d'Ikva): When there was Isur Kavu'a, and he knew that one piece is Chelev (one might have thought he does not bring a Korban).

Rashi (Kesuvos 22b DH b'Omeres and Yevamos 88b DH Bari): Asham Taluy is only for one who is unsure.

Rashi (Kerisus 17b DH Echad): If he knew that it is a Safek and he ate b'Mezid, he does not bring Asham Taluy.

Ritva (in Shitah Mekubetzes Kesuvos 22b DH Tanu): The Gemara says that when there are two pairs of contradictory witnesses, there is Asham Taluy for Bi'ah with her. This is like the opinion that the witnesses are Mevatel the previous Chazakah, so it is a Safek mid'Oraisa. According to the opinion that the witnesses cancel each other and it is a Safek mid'Rabanan, but mid'Oraisa the initial Chazakah still applies, there is no Asham Taluy, for she is either single or married! The Ri says that the Gemara was not precise. According to the opinion that it is a Safek mid'Oraisa, there is Asham Taluy. This is even like the opinion that requires two pieces. Since we can clarify her status, it is like eating one of two pieces. According to the opinion that it is a Safek mid'Rabanan, there is no Asham Taluy, but in any case it is forbidden.

Shitah Yeshanah (ibid.): Why does the Gemara says that they are Chayav Asham Taluy? This is only for Shogeg. Here, they are Mezid (before we answered that she married a witness, and also she is sure)! It means that it is an Aveirah for which one who is Shogeg brings an Asham Taluy.

R. Yonah (ibid.): One brings Asham Taluy if he ate one of two pieces (one of which was Chelev), even if he ate b'Mezid. Here they are Chayav Asham Taluy for each Bi'ah, for they do not know which set of witnesses tells the truth.

Me'iri (Kesuvos 22b DH Kevar): If a woman was established to be an Eshes Ish, and two witnesses said that she was widowed or divorced, and she married, and two other witnesses said that she was not widowed or divorced, there is Asham Taluy for Bi'os before the latter witnesses, for there was Chezkas Isur. There is no Asham Taluy for Bi'ah after the latter witnesses, for they are Mezid about a Safek Eshes Ish. If she married one of the former witnesses, and she says that it is clear to her that she was widowed or divorced, even though she does not explain how she knows, there is no (Chiyuv) Asham at all. Some explain that even when one has Bi'ah b'Mezid, knowing that it is a Safek, he is Chayav Asham. This is difficult. If so, why may she remain married after the latter witnesses came?!

Note: I do not understand his question. The first opinion exempts from Asham because he was Mezid, and does not merit protection from punishment, but not because it is a lighter Isur! Both hold like the Gemara concludes, that she does not leave when she married a witness and she is sure.

Poskim and Acharonim

Aruch l'Ner (Kerisus 26b DH uv'Din): Above, we concluded that regarding Asham Vadai, if he found out before Zerikah that he did not sin, it is buried. If the blood was thrown, it is burned. Also regarding Chatas ha'Of, we say that if he found out before Zerikah that he did not sin, it is buried. Asham Taluy is different. Since he was worried, he was Makdish it absolutely. This applies only to Asham Taluy, which is to protect from punishment. Chatas ha'Of is only to permit Kodshim, but not for protection.

Divrei Malkiel (1:86:5): Because one is worried, he is Makdish Asham Taluy absolutely. This is because it is to protect from punishment for entering a Safek. He should have avoided the Safek. We hold that one is liable only if there were two pieces. Even if later it is clarified that he did not sin, he wants to be Makdish it absolutely to atone for entering the Safek.

Teshuvas Chasam Sofer (CM 29): There is no difference whether or not one knew that one of the pieces is Isur. He was Shogeg, for he relied on his Da'as (knowledge).

Note: It is not clear to me what this means. Perhaps it means that for he relied on his Da'as to distinguish the Chelev from the Shumen, even though he is not so skilled and often errs about this. However, we cannot say so about Isurim that cannot be discerned from Heter, e.g. Pigul and an identical piece of Kosher meat!

The questioner in Shevet ha'Levi (2:37): In Nazir, Rashi connotes that at the time he ate he knew that it is a Safek. This is astounding. If so, how can he bring Asham Taluy? The Gemara itself connotes like this, for it thought that there is atonement only when there was only one piece. We must say that there are two kinds of Mezid (regarding a Safek). 1) He knows that there is a Safek, and is not concerned for the Isur. 2) He knows about the Safek, but is lenient and assumes that it is Heter.

Shevet ha'Levi: Rashi in Kerisus and the Chumash (Vayikra 5:17) explains that when he ate, he believed that both pieces are Heter. If he knew about the Safek, he does not bring a Korban. If he thought that he had a Siman or another way of knowing that it is Heter, he has no doubt. Rashi in Yevamos and Kesuvos says that he does not bring a Korban. The Ritva in Yevamos agrees to this principal, and we can say that the Ran agrees. We can say that the Gemara thought that when the Isur was fixed, this is not enough to get a Kaparah when he found out only afterwards. Perhaps "Rashi" on Nazir is not really Rashi (see R. Akiva Eiger, Nazir 2a. Also Rashi on Chumash explains like the opinion that expounds the verses in the way closest to the simple meaning, even if the Halachah does not follow this opinion - PF.) Rashi in Kesuvos connotes that the Gemara discussed a true Chiyuv Asham, even though he was aware of the Safek.

Question: If he did not know about the Safek when he ate, what was the Hava Amina that there is no atonement when there were two pieces? One brings a Korban if he never knew about the Isur, and found out later that he Vadai sinned, and all the more so if he never knew, and found out later that perhaps he sinned!

Answer (Shevet ha'Levi): If he will find out that he Vadai sinned, he will bring a Chatas. The Asham Taluy merely protects from punishment, like Rava taught. Perhaps this protection is only when there was only one piece, but not if Ikva Isura! (Then he was more negligent for entering the Safek.) Shitah Mekubetzes in Kesuvos explicitly brings opinions that Asham Taluy is even if he knew about the Safek. There, the Gemara said that one who marries her is Chayav Asham Taluy, before it answered that she married one of her witnesses. We must say that he says that he is sure that he died. The Chasam Sofer said simply that also Mezid brings Asham Taluy. He did not give a source, or explain how he learns Kerisus 18a, which explicitly exempts at least one kind of Mezid.