Let us continue explaining the principles we have learned, and clarify in which cases they apply to the Sanhedrin. We will begin with mitzvos that contain both a positive and a negative commandment together, and yet, even if the court ruled mistakenly about them, no par helem davar is brought. These are matters of the Beis HaMikdash, in which one who sins inadvertently is liable to a korban oleh v'yored rather than a fixed chatas.
A positive and a negative commandment regarding the Beis HaMikdash:
The negative commandment: a person who is tamei entering the Beis HaMikdash. If he does so deliberately his punishment is kareis, and therefore he is forbidden to enter; but if he does so inadvertently he is not liable to a fixed chatas, only to a korban oleh v'yored. For this reason there is no obligation to bring a par helem davar for a mistaken ruling in this matter, nor is the special bull of the kohen mashiach brought.
The positive commandment: one who discovers that he is tamei must leave the Azarah by the shortest and most direct route, and not by a roundabout way. If the Sanhedrin mistakenly ruled that he should leave by the roundabout route, then even though this is a mistaken ruling, it is not a case of helem davar. So too a kohen mashiach who ruled this way for himself and left by the longer route does not bring the special bull, since there is no fixed chatas here, only a korban oleh v'yored.
From here we also learn the law of a doubtful case: a person who does not know whether he entered the Mikdash while tamei - for example, there were two paths before him and he does not remember which one he took - or who does not know whether he left by the direct route or not by the direct route, does not bring an asham talui.
The reason is this: the rule is that wherever a certain transgression obligates a fixed chatas, a person brings an asham talui for a doubtful case - an offering that suspends judgment and protects him until the matter becomes clear, if it ever does, whether he actually sinned. But this rule does not apply to a korban oleh v'yored. And since one who enters the Mikdash while tamei inadvertently brings an oleh v'yored, no asham talui is brought for his doubt.
A positive and a negative commandment regarding niddah:
The Mishnah continues and teaches that there is a mitzvah that contains both a positive and a negative commandment together and nevertheless its law is different - namely, niddah. If the court ruled mistakenly and the entire community acted according to their ruling, a par helem davar is brought, and in the appropriate case twelve bulls on behalf of the community; and the same law applies to a kohen mashiach who ruled for himself and erred. And even in a doubtful case, if he does not know whether the matter occurred or not, he brings an asham talui. The reason for all of these is one: one who has relations with a niddah deliberately is punished with kareis, and inadvertently is liable to a fixed chatas.
How can there be a positive commandment regarding niddah?
The language of the Mishnah: "U'mitzvas lo saaseh shelo yavo el ha'niddah" - this is the negative side of the mitzvah: the general prohibition against having relations with a woman who has the status of a niddah, for which the punishment is kareis if done deliberately and a fixed chatas if done inadvertently. And the positive commandment within it is the obligation to separate from the niddah.
And this is how it works: if in the middle of relations it becomes known that the woman is a niddah, or she became a niddah at that moment, the husband may not separate immediately, since immediate separation still involves pleasure. Rather, he must wait until he becomes calm and the organ is lifeless and limp, and only then separate - so that he not derive pleasure from the act of separating itself.
It emerges that if he separated immediately, the stated liability applies to him: kareis if he did so deliberately, and a chatas if he did so inadvertently. This, then, is the scenario in which niddah contains both a positive and a negative commandment, and in which the Sanhedrin may become liable for a par helem davar or for the twelve bulls on behalf of the community; and likewise the kohen mashiach, who, if he ruled mistakenly for himself in this matter and acted according to his ruling inadvertently, must bring the special bull of the par kohen mashiach.