Parah, Chapter 12, Mishnah 4. This chapter takes us through the fine details of the actual sprinkling with the mei chatas, the water mixed with the ashes of the parah adumah, and here the Mishnah turns to a question of consequences: what happens when a person relies on a sprinkling that later turns out to have been worthless?
Begin with the opening case. "Hamazzeh mechalon shel rabbim": the sprinkling was performed on a person through a window belonging to the public, a station kept available for general use. "Veniknas lamikdash", and trusting that his taharah was now in order, he proceeded into the Mikdash. Only afterward did it come to light, "venimtzu hamayim pesulim", that the water applied to him had been invalid. His taharah, then, was never achieved at all, and his entry had taken place while he was still tamei. Even so, the Mishnah rules "patur": he owes no chatas.
Why is he exempt? Because a chatas is brought by one who sinned b'shogeg, unintentionally but carelessly. This man is not classified as a shogeg but as an ones, someone whom circumstances forced into the transgression. Water standing at a public window for the community's use carries a presumption of being fit for sprinkling. He had every right to rely on it, and he cannot be faulted for what he could not have known.
The second case: "mechalon shel yachid veniknas lamikdash venimtzu hamayim pesulim, chayav". Here the water came from a private window, an individual's own supply, and after he entered the Mikdash the water was discovered to be pasul. In this case he is chayav to bring a chatas, because he is a genuine shogeg. Private water carries no communal presumption of validity, and it was on him to look into the matter and verify that the water he was purifying himself with was fit before relying on it to enter the sanctuary.
Next comes an exception in favor of the Kohen Gadol: "aval Kohen Gadol". Here it makes no difference whose water was used. Whether the source was "mechalon shel yachid", a window owned by an individual, or instead "mechalon shel rabbim", one belonging to the community, in both situations he is "patur". The Mishnah supplies the reason in a brief clause: "she'ein Kohen Gadol chayav", the Kohen Gadol bears no liability, "al bi'as hamikdash", for coming into the sanctuary. He occupies a category of his own in relation to the Mikdash, so the korban chatas that an ordinary Yisrael would owe for entering in a state of tumah does not attach to him.
The closing line of the Mishnah sketches a scene of ordinary foot traffic near these windows. "Machalikin hayu": the ground would turn slick underfoot. And where? "Lifnei chalon shel rabbim", right outside the public window, since sprinklings were performed there without interruption and the drippings gathered on the earth below. What did passersby do about it? "Vedorsin velo nimna'in", they stepped straight across the wet spot and never held themselves back or went around, taking no precaution against that mud as a source of tumah.
What justified such confidence? "Mipnei she'amru", because a ruling had already been established: "mei chatas she'asu mitzvasan", once the chatas waters have carried out the very task for which they were prepared, "einan metam'in", they no longer transmit tumah. The moisture soaking that patch of ground was spent water; its mitzvah had already been fulfilled, and so whoever walked through it could go on his way without hesitation.
The Mishnah has thus given us two lessons side by side: a person who leans on the community's water is treated as one caught by circumstance rather than as a careless sinner, while a person leaning on his own private supply carries the responsibility to check it; and the very water that purifies while it is still fit for its mitzvah has no power to defile once that mitzvah has been carried out.