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The Talmud says:

It is better to transgress unintentionally than intentionally. Betzah 30a

The Midrash adds:

Rabbi Shim'on ben Halafta said: One who studies the commandments of the Torah, but does not fulfill them, is punished more severely than one who never studied at all. Deuteronomy R. 7:4

I remember reading, in commentaries on the above, that Jewish law says that, in minor cases, if a rabbi determines that the people are going to do a certain wrong thing no matter what he says, it is better not to inform them that they are breaking a commandment, because by teaching them that, they would then break the commandment deliberately. Is this correct and what is the reference?

The gemara on Shabbat 148b:

ואמר ליה רבא בר רב חנן לאביי תנן לא מספקין ולא מטפחין ולא מרקדין ביו"ט וקא חזינן דעבדין ולא אמרינן להו ולא מידי ולטעמיך הא דאמר רבא לא ליתיב איניש אפומא דלחייא דילמא מיגנדר ליה חפץ ואתי לאיתויי והא קא חזינן נמי דמותבי חצבי ויתבן אפומא דמבואה ולא אמרינן להו ולא מידי אלא הנח לישראל מוטב שיהו שוגגין ואל יהו מזידין סבור מינה הנ"מ בדרבנן אבל בדאורייתא לא ולא היא ל"ש בדרבנן ול"ש בדאורייתא דהא תוספת דיוה"כ דאורייתא היא וקא חזינן להו דקאכלי ושתו עד שתחשך ולא אמרי' להו ולא מידי:‏

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And Rava bar Rav Ḥanan said to Abaye: Did we not learn in a mishna that one may not clap hands, or clap one’s hand against one’s body, or dance on a Festival? And we see, however, that people do these things, and we do not say anything to stop them. Abaye responded: And according to your reasoning, what about this halakha that Rava said: One may not sit on Shabbat at the entrance of a private alleyway next to the post, which delineates its boundaries, lest an object roll away into the public domain and he come to bring it back? And yet we see that women put down their jugs and sit at the entrance of the alleyway, and we do not say anything to stop them. Rather, in these matters we rely on a different principle: Leave the Jewish people alone, and do not rebuke them. It is better that they be unwitting in their halakhic violations and that they not be intentional sinners, for if they are told about these prohibitions they may not listen anyway.
There were those who understood from this statement that this halakha applies only to rabbinic prohibitions but not to Torah prohibitions, with regard to which we must certainly reprimand transgressors. However, that is not so. There is no difference between rabbinic prohibitions and Torah prohibitions. In both cases one does not reprimand those who violate unwittingly and would not listen to the reprimand. For the requirement of adding to Yom Kippur by beginning the fast while it is still day is from the Torah, and we see women who eat and drink on the eve of Yom Kippur up until nightfall, and we do not say anything to them. Thus, this rule, which applies to rabbinic prohibitions, applies to Torah prohibitions as well>
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Rema to Orach Chaim 608:2 limits the application of this principle to any law which is not explicit in the Torah (even if it is treated as a biblical law) such as the requirement to begin observing Yom Kippur a short while before sunset.