Parah, Chapter 12, Mishnah 3. This chapter deals with the ezov and with the act of sprinkling the mei chatas. Our Mishnah asks what happens when a man dips the hyssop with one target in mind and the water ends up somewhere else: must he go back and dip again, or may he continue with the water still held in the hyssop?
The opening case is one where the water lands on something outside the realm of tumah altogether. He set his intention on an object that can contract tumah, a "davar shehu mekabel tumah", yet the drops actually struck an object that can never become tamei. The ruling is "lo yishneh": as long as the ezov still holds a sufficient quantity of water for a valid sprinkling, there is no need for a fresh dipping, and he proceeds to sprinkle with whatever remains in his hand.
Turn the case around and the halachah changes. His intention was directed at an object beyond the reach of tumah, and the drops instead reached an object that does contract tumah. Now the Mishnah says "yishneh": whatever water is still lodged in the ezov may no longer be used, and he must return to the vessel and dip once more before sprinkling.
The same pair is then presented with living targets. Where he was aiming at a person and the drops reached an animal, leftover water in the ezov is still usable and he continues sprinkling with it, "lo yishneh". Where he was aiming at an animal and the drops reached a person, the water remaining in the ezov will not do, "yishneh", and he dips anew.
The Mishnah ends with a word about the runoff of these sprinklings. The drops that fall away, the "mayim hanotfim", are "ksherim": they have lost none of their standing. And from that very validity the Mishnah draws its conclusion: since they still count as mei chatas, they also transmit the tumah that mei chatas transmits, no differently from the water resting in the vessel itself.