והלא מצרף רבי שמעון היא דאמר דבר שאין מתכוין מותר מתקיף לה אביי מידי מיחם שפינה ממנו מים קתני מיחם שפינהו קתני
The Gemara questions this leniency: By putting cold water into the urn, doesn’t it harden the vessel? Cold water poured into a heated metal vessel reinforces the vessel. It is one of the stages in the labor of a blacksmith. How is it permitted to do something similar on Shabbat? The Gemara answers: This mishna is in accordance with the opinion of Rabbi Shimon, who stated a principle with regard to the laws of Shabbat: An unintentional act, i.e., a permitted action from which a prohibited labor inadvertently ensues, is permitted. Here too, his intention was to warm the water, not to reinforce the vessel. Abaye strongly objects to this explanation: Does it say in the mishna: An urn from which water was emptied? That would indicate that he sought to fill the vessel with other water and warm up that water. Rather, an urn that was removed was taught in the mishna, meaning that the urn was removed from the fire, not that the water was emptied from it.
אלא אמר אביי הכי קאמר המיחם שפינהו ויש בו מים חמין לא יתן לתוכו מים מועטין בשביל שיחומו אבל נותן לתוכו מים מרובים כדי להפשירן ומיחם שפינה ממנו מים לא יתן לתוכו מים כל עיקר מפני שמצרף ורבי יהודה היא דאמר דבר שאין מתכוין אסור
Rather, Abaye said this is what the mishna is saying: An urn that was removed from the fire and contains hot water, one may not place a small quantity of water in it so that the water will become hot; rather, one may place a large quantity of water in it so that the water will become warm. And with regard to an urn from which water was removed; one may not place any water into it because he hardens the vessel by placing cold water into a hot vessel. And, according to this explanation, our mishna is in accordance with the opinion of Rabbi Yehuda, who said that an unintentional act from which a prohibited labor inadvertently ensues is prohibited on Shabbat.
אמר רב לא שנו אלא להפשיר אבל לצרף אסור ושמואל אמר אפילו לצרף נמי מותר לצרף לכתחילה מי שרי אלא אי איתמר הכי איתמר אמר רב לא שנו אלא שיעור להפשיר אבל שיעור לצרף אסור ושמואל אמר אפילו שיעור לצרף
With regard to the matter itself, Rav said: They taught that one is permitted to place cold water into an urn with hot water after it was removed from the fire, when his intention is only to warm the cold water. However, if he did this in order to harden the vessel, it is prohibited. And Shmuel said: Even if he does so in order to harden the vessel, it is also permitted. The Gemara wonders: Is hardening permitted ab initio? Isn’t it a full-fledged prohibited labor on Shabbat? Rather, if the dispute between Rav and Shmuel was stated, it was stated as follows. Rav said: They taught that it is permitted to add water only in a measure that is sufficient to warm the water, i.e., to only partially fill the vessel. However, filling it completely with a measure sufficient to harden the vessel is prohibited. And Shmuel said: Even a measure sufficient to harden the vessel