Pesachim 51 - Wrong customs
Earlier we learned that one should follow the customs of the place he came to, and not deviate from them, in order to avoid disagreements. However, here is an example of a custom that was adopted by the unlearned masses on their own: the people of Chozai separated the kohen’s portion (challah) from dough made of rice. This is not required at all, and Rav Yosef wanted to send a non-kohen who would eat such "challah" in front of them, thus proving that their custom had no validity. Abaye wanted to stop him, using our previous lesson, that one should not deviate from an existing custom. However, Rav Yosef replied that this lesson was about unlearned masses.
It turns out that there are two levels of ignorance. Totally unlearned masses should keep their wrong customs, because once you start abrogating these customs, they will eventually confuse all the laws. However, a custom which was initially adopted with the consent of a Sage - such custom, if found erroneous, can later on be argued against, because the group of people with some education will understand the difference: they will accept the eradication of the wrong custom but keep the other rules intact.
Here is an example. The two sons of Rabban Gamliel bathed together in the city of Kabul. The populace was amazed, saying, "We have not seen such behavior ever in our lives." At which one of the brothers left the bath, but he did not wish to tell them, "You are wrong, two brothers are allowed to bathe together, it is only that two brothers-in-law are not (because this may lead one to wrong thoughts about his sister)." Why didn’t they abrogate the custom? - Because the people of Kabul, although somewhat learned, were rarely visited by Sages.
Art: Trouville, Bathing Hour by Eugène Boudin