Mishnah Niddah 6:13: Regarding a woman who saw a [blood] stain [on her clothing], she [with regarding to her counting of her days of impurity] is spoiled [i.e. she can no longer rely on her count], and she must take into consideration the possibility that there was a discharge [that day and the previous two days, rendering her a zavah and thereby impure for another seven days at least], according to Rabbi Meir. But the Sages say: [blood] stains do not contain discharge [and thus she need not suspect that she may be a zavah and therefore impure].

הָרוֹאָה כֶתֶם, הֲרֵי זוֹ מְקֻלְקֶלֶת, וְחוֹשֶׁשֶׁת מִשּׁוּם זוֹב, דִּבְרֵי רַבִּי מֵאִיר. וַחֲכָמִים אוֹמְרִים, אֵין בַּכְּתָמִים מִשּׁוּם זוֹב:

Mishnah Niddah 6:14: Regarding a woman who saw [a discharge of blood] on the eleventh day [i.e. her last day of counting, after which she should be pure] at twilight, or at the beginning of her being a niddah [a woman who has menstrual discharges which render her impure] and at the end of her being a niddah or at the beginning of the [subsequent] period of [potential] genital discharge and the end of that period of discharge [i.e. at twilight on what is either the first day of her being a niddah or the last day of the subsequent period of discharge during which she counts seven clean days, or at twilight on what is either the last day of her being a niddah or the first day of the subsequent period of potential discharge], or [if she saw blood] at the end of the fortieth day [after the birth] of a male, or at the end of the eightieth day [after the birth] of a female, at twilight in all these cases, they are thereby [assumed to be] mistaken [in their counting, and must therefore treat each case stringently]. Rabbi Yehoshua said: now that you have remedied [the situation for] the foolish women [who have mistaken their count], remedy [the situation for] the intelligent ones [who know how to count, as well, since they too may err in their counting].

הָרוֹאָה יוֹם אַחַד עָשָׂר בֵּין הַשְּׁמָשׁוֹת, תְּחִלַּת נִדָּה וְסוֹף נִדָּה, תְּחִלַּת זִיבָה וְסוֹף זִיבָה, יוֹם אַרְבָּעִים לַזָּכָר וְיוֹם שְׁמוֹנִים לַנְּקֵבָה, בֵּין הַשְּׁמָשׁוֹת לְכֻלָּן, הֲרֵי אֵלּוּ טוֹעוֹת. אָמַר רַבִּי יְהוֹשֻׁעַ, עַד שֶׁאַתֶּם מְתַקְּנִים אֶת הַשּׁוֹטוֹת, תַּקְּנוּ אֶת הַפִּקְחוֹת: