Taharos, Chapter 10, Mishnah 6. This chapter has been dealing with grapes, wine and the winepress, and with the ways in which liquid can carry tumah into the wine. Our mishnah closes with a case of doubt: something small and unnoticed may have landed in the wine, and may not have.
The case
The mishnah says: "Mi shehayah omeid umedaber al sefas habor", a person was standing and talking at the rim of the wine pit. The Bartenura points out that this pit is built of plaster, and a plastered pit is not itself susceptible to tumah, so the question here is only about the wine collecting inside it.
"Venitzah tzinorah mipiv", and as he spoke, a spray of spittle flew out of his mouth. If the man is tamei, his spittle carries tumah with it. Had we seen it land in the grapes, the matter would be clear: the wine would be tamei. Had we seen it land outside the pit, it would equally be clear that the wine remains tahor.
The ruling
But our case is neither. "Safek higi'a labor, safek lo higi'a", it is a genuine doubt whether the spittle reached the pit or not. And the mishnah rules: "sefeiko tahor", in a case of doubt like this, the wine is tahor. We assume that the spittle most likely struck the outer edge of the pit rather than traveling into the wine itself.
A quiet ending to the chapter, and a reminder of how the halachos of tumah and taharah treat a doubt: where nothing definite has been established, we do not go looking for tumah.