Niddah, Chapter 6, Mishnah 14. This mishnah takes up the case of a woman who finds a stain of blood on her garment during the days in which a sighting of blood could render her a zavah, and it records a dispute about whether such a stain disturbs her count.
The setting: the eleven days of zivah
The Torah's cycle for a woman divides into two stretches. First come the seven days of niddah. Following them come eleven days known as the yemei zivah, the days of zivah. Blood seen during those eleven days does not make her a niddah; it carries the potential to make her a zavah. That is why a woman in those days must keep careful track: she needs to know exactly which day of the eleven she is standing on, and exactly on which days she saw.
The case of the mishnah
"Haroah kesem": a woman in those eleven days discovers a stain on her undergarment. Rabbi Meir rules, "harei zo mekulkeles vechosheshes mishum zov": her count is thrown into disarray, and she must reckon with the possibility of zivah.
Why is her count ruined? Because a stain, unlike a sighting felt and identified at the moment it occurs, testifies to nothing about when the blood emerged. Perhaps it came today. Perhaps yesterday. Perhaps two days ago, or three. Since she cannot pin the blood to a particular day, she no longer knows with clarity where she stands in the eleven days, and her count is confused.
Rabbi Meir goes a step further. A single stain can carry a suspicion of full zivah. A woman becomes a zavah only through blood on three consecutive days, but Rabbi Meir entertains the possibility that this one stain is the accumulated product of exactly that: blood on the first day, blood again on the second, blood again on the third, with the woman noticing nothing until she finally examined her garment on the third day. One stain, therefore, may already leave her a possible zavah.
The view of the Chachamim
The Chachamim disagree: "ein baksamim mishum zov," stains do not generate a concern of zivah. Their reasoning rests on the requirement of three consecutive days. A woman is a zavah only when blood appears on three days in a row, and a lone stain is not treated as though it were secretly assembled out of three separate sightings.
Where three stains appear, however, the Chachamim do take zivah into account. Three separate marks can be attributed to three separate days, one mark per day, and she is then treated as a possible zavah. What they refuse to assume is that a solitary stain conceals within it three days of bleeding, and so a lone mark leaves her free of any zivah concern.