Shevuos, Chapter Two, Mishnah 4. This mishnah continues the idea discussed in the previous mishnah, which deals with rulings - cases where the Great Beis Din errs in a ruling involving a prohibition punishable by kareis.
In Mishnah 3 we learned that when a person enters the Beis Hamikdash while in a state of tumah, or eats sacred food while in a state of tumah, even though this carries the penalty of kareis, the beis din does not bring a par he'elem davar of the congregation for its error - the standard offering brought for a mistaken ruling. The reason: the matter of tumah of the Mikdash and its sacred foods does not obligate a fixed chatas but rather a korban oleh v'yored (a sliding-scale offering), whereas the par he'elem davar of the congregation is brought only when an individual would be obligated in a fixed chatas.
The previous mishnah also dealt with a particular case: a person who entered the Beis Hamikdash in a state of purity and became tamei while inside - he has a positive commandment to leave the Beis Hamikdash as quickly as possible.
The positive commandment connected with niddah:
Our mishnah, by contrast, deals with what is called the "positive commandment connected with niddah": a woman who became a niddah in the middle of marital relations, what is the law? The wording of the mishnah: "V'eizohi hi mitzvas asei shebanidah shechayavin alehah?" - that is, what is the positive commandment bound up with the prohibition of niddah, concerning which an erroneous ruling of the beis din would obligate a par he'elem davar of the congregation? For here we are dealing with a prohibition of kareis for which the husband brings a fixed chatas, and one may say that both of them bring a fixed chatas.
"Hayah mishameish im hatehorah" - he was in the midst of marital relations with a woman who at the beginning of the act was pure and not a niddah. "V'amrah lo nitmeisi" - in the middle of the act she told him that she felt she had become a niddah. "U'feirash miyad" - and he separated from her immediately - he is liable.
And why? "Sheyetziaso hana'ah lo kiviaso" - the immediate withdrawal is pleasurable to him just as the initial entry was pleasurable. Therefore he may not separate immediately; rather, he must wait until the organ subsides, and only afterward separate.
It emerges that if the beis din had ruled that he must separate immediately, they would be obligated to bring a par he'elem davar of the congregation. For the prohibition of niddah carries the penalty of kareis, for which an individual is obligated in a fixed chatas offering, and wherever an individual becomes obligated in a fixed chatas, the beis din becomes obligated for its error in a par he'elem davar of the congregation.
In summary: this mishnah places two positive commandments side by side:
One who leaves the Mikdash: tumah of the Mikdash and its sacred foods obligates a korban oleh v'yored, and therefore the beis din is not liable for it with a par he'elem davar of the congregation.
The positive commandment connected with niddah: the prohibition of niddah carries kareis, which obligates a fixed chatas, and therefore an erroneous ruling of the beis din that he should separate immediately - instead of waiting until the organ subsides - obligates a par he'elem davar of the congregation.