We are now at Mishnah 8 in the eighth chapter of Tractate Zevachim, which deals with the blood of unblemished animals that became mixed with the blood of blemished animals: blood meant to go up on the altar, from an unblemished animal that has no blemish, which became mixed with the blood of a blemished animal or animals.
The Torah's Prohibition regarding the Blood of a Blemished Animal:
The Torah explicitly forbids applying the blood of a blemished animal on the altar, as it states: "Lo takrivu laShem" - "You shall not offer to Hashem" (Vayikra 22:24), and from here we learn the prohibition against applying the blood of a blemished animal. Therefore, everyone agrees in this case that the mixture is poured into the canal. Even Rabbi Eliezer holds that one may not apply this blood as if it were water, since the Torah completely forbade its application. Therefore, the entire mixture is poured into the water canal that passes through the Temple courtyard.
Here too, we wait until after sunset, since until then the mixture still contains valid blood which is forbidden to pour out, as it includes something permitted for application.
"Kos bekosos" - A cup among cups:
Here the Mishnah presents a mixture that is not a liquid-in-liquid type - two liquids mixing together, where every sample of them has a bit of the permitted and a bit of the forbidden. We are talking about separate cups: suppose we have three cups before us, two containing valid blood and one containing the blood of the blemished animal. The cups became mixed up with each other and we do not know which is which, and therefore none of them may be brought up.
Nullification in a Majority and a Significant Entity:
By Torah law, the usual rule is nullification in a majority: in a mixture, all the components take on the status of the majority. Since two cups are valid and one is invalid, all three take on the status of the majority, and they should all be permitted by Torah law. However, since one cup is completely forbidden to be offered, and a separate cup of blood is also considered a significant entity, it is not nullified by Rabbinic law.
Attributing to an Offered Cup:
This is the exact same structure we saw in Mishnah 5. Everyone agrees that at this point a person is at a dead end: the forbidden cup may not be applied to the altar, and since he does not know which is the forbidden one, he cannot apply any of them, and the blood will remain until it becomes invalid by being left overnight. But if one of the cups was offered by mistake, before the question arose that there is a mixture here - can we now attribute it and say that the cup that went up is the one that was not supposed to go up, and since it already went up on the altar, everything remaining is permitted and even required to be offered?
The reasoning for this, as we already said in Mishnah 5, is that the rationale of attribution in and of itself certainly would not be enough. However, when two cups remain, there is certainly a mitzvah of application in one of them; and even if there is a transgression in the other, it is only a Rabbinic transgression at most. Beyond this, there is reason to say that there is no problem here at all, since it is possible that the cup that was prohibited Rabbinically is the one that was already placed on the altar.
Rabbi Eliezer: Permits placing the pair of cups at once, consistent with his opinion in Mishnah 5. For he is certainly fulfilling a mitzvah in doing so, it is possible that he is not transgressing at all, and in the worst case scenario it is only a Rabbinic transgression.
The Sages: Forbid it, just as they forbade it in Mishnah 5. Since by Rabbinic law the cup is not nullified, we have before us a mixture of something that might be forbidden, even if only Rabbinically, and nothing from it may be applied to the altar. We wait until after sunset, and everything is poured into the water canal in the Temple courtyard.
Why is the law repeated regarding blood?
The rules here are identical to the rules stated in Mishnah 5, and seemingly, what room is there to differentiate between the two cases - between limbs and blood? The answer is that there was room to think that the blood is more significant, because it is the essence of the sacrifice: the blood sprinkled on the altar, whether it effects the atonement or not, is what determines whether the sacrifice has been performed or not. Because of this, one might have thought that there is an additional reason with blood to be lenient or stringent, and therefore the Mishnah clarifies that the positions regarding blood are identical to the positions stated regarding limbs: in that unique scenario where one cup has already been offered, we can attribute it, and according to Rabbi Eliezer it is permitted to offer two simultaneously after that action, and according to the Sages they may not be offered at all.
The language of the Mishnah:
"Kos bekosos" - one invalid cup that became mixed with valid cups, with at least three cups in total, where each cup is entirely permitted or entirely forbidden, without being physically mixed together.
"Rabbi Eliezer omer: im karav kos echad - yikrevu kol hakosos" - if one cup was already offered by mistake, what is done is done, and therefore the remaining ones are also placed on the altar: a mitzvah is certainly being fulfilled here, and at most it is a rabbinic prohibition, and we can attribute that the cup which was offered is the one that was rabbinically forbidden to begin with.
"Vachachamim omrim: afilu krevu kulan chutz me'echad mehen - yishafech la'amah" - even if all were offered and only one cup remained, such as if there were a hundred cups and this one was left, we cannot assume it is the valid one. This majority is not enough to attribute it, and therefore it is poured out into the water channel in the Temple courtyard after sunset.
The condition explained in Mishnah 5:
That same condition applies here as well: if only one cup remained, even Rabbi Eliezer concedes that it may not be offered. Two cups must remain, so that it is certain a mitzvah is fulfilled; whereas when one remains, it is possible that it itself is the forbidden cup. Therefore, the simple understanding of the Mishnah here is that one pair of cups remains. The Sages say that even with one pair against a hundred, they may not be offered, and Rabbi Eliezer is lenient.
Practical halachah: The halachah follows the Sages, as we treat this rabbinic prohibition with full seriousness. Therefore, we do not make assumptions and do not attribute anything, and we say that in any mixture of forbidden and permitted - since the rabbinically forbidden parts cannot be offered, none of it may be offered at all.
In summary: In this Mishnah, we learned that the blood of unblemished animals that became mixed with the blood of blemished animals is poured into the water channel according to everyone, because the Torah explicitly forbade offering the blood of a blemished animal. We discussed a "kos bekosos" - a mixture of separate cups, which by Torah law is nullified in a majority, but rabbinically is not nullified because a cup of blood is considered a significant item. We saw the dispute between Rabbi Eliezer and the Sages regarding attribution after one cup was mistakenly offered, the condition that specifically two cups remain, and the ruling following the Sages that we do not make attributions and do not offer from the mixture at all.