Parah, Chapter 7, Mishnah 6. Chapter seven turns again and again on one idea: from the moment a person begins drawing water for the chatas, the purifying waters of the parah adumah, until the ashes are added, he may not occupy himself with any other melachah. Our mishnah asks about an act that looks entirely innocent, and shows how much can hang on whether a man takes the direct road or turns aside.
Here are the mishnah's words, phrase by phrase: "Hamolich es hachevel beyado" (one who transports a rope while he walks), "ledarko", meaning that his path passes the owner's door in any event, "kasher", the drawn water retains its validity. "Veshelo ledarko", where he must swing away from his route, "pasul", the water is disqualified.
Picture the case. A man borrowed a rope from his fellow and used it to lower the pitcher into the spring and draw up the water. Now, with the pitcher already resting on his shoulder, he keeps the rope in his hand in order to return it to its owner. If the owner's house lies along the route he is walking anyway, with no detour, carrying the rope is not reckoned as a melachah at all, and the water of the chatas remains fit.
But if returning the rope takes him off his path, the picture changes. Carrying an object by way of a detour is an act of labor in its own right, an errand he has undertaken while the mitzvah water is in his charge, and the water is thereby rendered unfit.
The mishnah closes with a remarkable historical note: "Zeh halach leYavneh sheloshah mo'ados, uvamo'ed hashelishi hichshiru lo hora'as sha'ah." This man made the journey to Yavneh during three separate pilgrimage seasons, and at the last of those visits the Sages validated his water by way of a ruling issued for that moment alone.
The situation under discussion was one where the rope had indeed been taken along a detour. A talmid chacham set out for Yavneh, the seat of the Sanhedrin once it had been exiled from Yerushalayim, in order to place the she'eilah before the Chachamim. The tradition surrounding this halachah tells that the Jewish community of Asya presented themselves at Yavneh over the course of three yamim tovim in an effort to obtain a ruling, and only at the last of those appearances did the beis din pronounce the water valid. What they issued was a hora'as sha'ah, a dispensation extended for that particular moment because the need was urgent.
The words the mishnah chooses are careful, and they preserve both halves of the truth. By the standing law, a man who takes his rope out of his way while bearing the water of the chatas has invalidated it. What happened at Yavneh was an emergency dispensation, granted once, in a time of need, by the great court of the generation, and it does not unseat the rule with which the mishnah opens: along his way, fit; out of his way, unfit.