Yevamot 15B

הרואה אומר: לאפושי מיא הוא דקא עביד.

anyone watching would say that he did it to increase the water flow, and he did not intend to change the halakha. Rather, they would think that he widened the water trough only because there was not enough water flow.

תא שמע, דאמר רבי אלעזר בר צדוק: כשהייתי לומד תורה אצל רבי יוחנן החורני, ראיתי שהיה אוכל פת חריבה במלח בשני בצורת. באתי והודעתי את אבא, אמר לי: הולך לו זיתים, והולכתי לו. ראה אותן שהן לחין, אמר לי: אין אני אוכל זיתים.

§ The Talmud cites another relevant source. Come and hear, as R' Elazar bar Tzadok said: When I studied Torah with R' Yoḥanan the Ḥorani, who was a disciple of Beit Shammai, I saw that he would eat dry bread in salt in years of drought. I went and informed my father of his meager meal, and he said to me: Take olives for him, and I took for him some olives. R' Yoḥanan saw that they were moist with the liquid that oozes from olives, which renders them susceptible to ritual impurity. Concerned that they had already become ritually impure, he said to me: I do not eat olives. He spoke politely and stated that he did not eat olives because he did not wish to embarrass his benefactor by disclosing that he feared they had become ritually impure.

באתי והודעתי את אבא. אמר לי: לך ואמור לו, חבית נקובה היתה אלא שסתמוה שמרים. ותנן: חבית של זיתים מגולגלים, בית שמאי אומרים: אין צריכה לנקב.

R' Elazar bar Tzadok continued his account. I went and informed my father. He said to me: Go and say to him that the jug containing the olives was perforated, but it was clogged by sediments in the oil and therefore some moisture remained inside. However, due to the presence of the hole, the olives were not rendered susceptible to ritual impurity. And we learned this in a Mishnah: With regard to a jug of preserved olives, Beit Shammai say it does not need to be perforated, as the juice issuing from the olives does not render them susceptible to ritual impurity.