Niddah, Chapter 10, Mishnah 8. This mishnah belongs to the series of disputes between Beis Shammai and Beis Hillel that closes the tractate. Here the subject is a woman who saw blood during the eleven days of zivah and then went to the mikveh sooner than the halachah allows, and what follows from that both for tumah and for the korban.
Before the case itself, the assumptions behind it. The cycle in this area of halachah falls into two stretches: seven days set aside for niddah, and then a further eleven in which any bleeding is reckoned against zivah. Blood appearing in that later stretch cannot be classified as the blood of a niddah; it must be handled as the blood of a zavah. One day's sighting makes her what the mishnah calls shomeres yom keneged yom: she guards the day that follows, watching whether the bleeding returns. Should it come back on a second day and again on a third, three days running, she has the full status of a zavah gedolah, with all the stringencies that status carries.
The mishnah's first case is a woman who saw blood specifically on the last of those days: "haro'ah yom achad asar", one who sees on the eleventh day. By right she is a shomeres yom keneged yom, and she should have waited out the following day to see whether more blood would come and carry her further toward the status of a zavah gedolah. She did not do so. "Vetavlah le'erev veshimshah": that same evening she immersed in the mikveh and lived with her husband.
Beis Shammai omrim: "metam'in mishkav umoshav vechayavin bekorban." As they see it, her zavah status stands and his is that of a bo'el zavah, a man who has had relations with a zavah. The upshot is twofold: whatever the two of them lie on or sit on becomes tamei through them, and a korban chatas falls due from each of them.
Beis Hillel omrim: "peturin min hakorban." Beis Hillel does not deny that the couple acted as they should not have. Nevertheless, once the eleventh day has passed her days of zivah are genuinely finished, and there is no real possibility of her becoming a zavah gedolah from that seeing. To obligate a korban chatas here would therefore raise the problem of bringing chullin, an ordinary animal, into the Beis Hamikdash, and for that reason Beis Hillel will not impose the obligation.
The mishnah then moves to a second, milder case: "tavlah beyom shele'acharav veshimshah es beisah, ve'achar kach ra'asah." Here she waited and immersed on the following day, the twelfth day, after the days of zivah, then lived with her husband, and only afterward saw blood.
Beis Shammai omrim that they "metam'in mishkav umoshav" and yet are "peturin min hakorban." Immersion was not yet open to her at that point, nor was living with her husband, so in Beis Shammai's eyes she still carries the status of a zavah and he that of a bo'el zavah, and anything they lie or sit upon picks up tumah from them. On the korban, however, Beis Shammai releases them here: no chatas is owed.
Uveis Hillel omrim: "harei zeh gargaran." Beis Hillel answers by characterizing the man rather than by issuing a ruling in tumah. He is called a glutton: too impatient to wait, exploiting a technical opening in the law to gratify himself. In practical terms, though, nothing follows from what they did. Their mishkav and moshav stay clean of tumah, and no korban is owed.
The mishnah ends with the case in which the two houses do not divide. "Umodim", they concur, "baro'ah besoch achad asar yom, vetavlah le'erev veshimshah, shemetam'in mishkav umoshav vechayavin bekorban." Where the bleeding fell inside the eleven day stretch and not on its final day, and she went to the mikveh that same evening and was with her husband, there is no argument. Her zivah days are still running and further sightings are still ahead of her, and so whatever the two of them lie on or sit on becomes tamei, and a korban is due from each.
And if, in that same situation, "tavlah beyom shele'acharav veshimshah", she waited and immersed on the following day and then lived with her husband, the mishnah rules: "harei zo tarbus ra'ah, umaga'an uvi'ilosan teluyin." This is bad practice, conduct that ought not to be adopted, and the halachic outcome is left hanging. Whatever they touched and the relations between them remain suspended, awaiting the rest of that day on which she immersed. If she sees blood later that day, her immersion is undone retroactively, whatever she touched is retroactively tamei, and she and her husband are liable to a korban chatas for having been together while she was a zavah. If no blood appears, nothing is tamei and nothing is owed. What was done in haste is left, until the day ends, in doubt.