Gemara

(Mishnah): If a member of the Beis Din or a Chacham qualified to give Hora'ah knew that Beis Din erred, and transgressed, he is liable (brings a Korban), for he did not rely on Beis Din.

2b (Rava): It teaches 'a member of the Beis Din' and also 'a Chacham qualified to give Hora'ah' to teach that he is liable even if he is not Gamir (he did not learn the matter), but he is Savir (he can learn one matter from another through analysis), or if he is Gamir, but he is not Savir.

Question (Abaye): A Chacham qualified to give Hora'ah must be Gamir and Savir!

Answer (Rava): True! The extra clause obligates even if he is only Gamir or Savir.

Even though he knew the law, he erred about the Mitzvah to heed Divrei Chachamim.

4b (Mishnah): If Beis Din erred, and the people sinned b'Mezid, they are exempt.

Sifri (on Devarim 17:11): (It says "do not veer from what they (the Great Sanhedrin) tell you, left or right.") This is even if he tells you that right is left and left is right, and all the more so if he tells you that right is right and left is left.

Rishonim

Rambam (Hilchos Shegagos 13:5): If Beis Din permitted all Chelev of the stomach, and someone else knew that they erred, and ate it due to their ruling, for he thought that it is a Mitzvah to heed Chachamim even if they err, he brings a Korban. This is if he is a Chacham or Talmid qualified to rule. If he is an Am ha'Aretz (ignoramus) he is exempt, for his knowledge of Isur is not definite.

Ri Korkus: It is clear from the Gemara that if he is not Gamir and not Savir, even if he knew that they erred, he is not called one who relied on himself (so he is exempt).

Lechem Mishneh: Above (13:1), the Rambam said that an individual who sinned due to following Beis Din brings a Korban! We must say that here he means that if the majority of Yisrael sinned and they bring Par Helam Davar, he is exempt from bringing his own Korban.

Rambam (6): If Beis Din erred and the Tzibur knew this, they should not accept this. If they acted based on the ruling, they do not bring a Korban, for they are Mezidim. They knew that it was a mistake, and it is improper to follow it.

Ri Korkus: The Rambam (Halachah 5) exempts an Am ha'Aretz who knew that Beis Din erred but followed them (for he should not rely on himself). That refers to an individual. Here, the Tzibur knew. This is Vadai knowledge. They are Muchzak, and the law is clear to them (so they should not follow Beis Din). Or, perhaps we assume that there are Talmidim (qualified to give Hora'ah) among them. The first Perush is true.

Rambam (Hilchos Mamrim 1:2): Anyone who does not do like the ruling of the Great Sanhedrin transgresses the Lav "Lo Sasur (do not deviate from what Chachamim say)". One is not lashed for this, for one can be killed for it, i.e. a Zaken Mamrei (without rules unlike them). This applies to traditions, i.e. Torah she'Ba'al Peh, things expounded through the Midos, and fences made for the Torah according to the needs of the time, i.e. decrees, enactments and customs. It is a Mitzvas Aseh to obey all of these, and a Lav to transgress.

Lechem Mishneh: The Ramban (Sefer ha'Mitzvos, Shoresh 1) asked, if so, there is a Lav and Aseh to fulfill all mid'Rabanan laws. We should be more stringent about mid'Rabanan laws than mid'Oraisa laws! Kiryat Sefer answered that Chachamim stipulated that one who transgresses their words is not like one who transgresses Torah, and that one may be lenient about a Safek. The Rambam says that (even so) there is a Lav and Aseh, i.e. for a Zaken Mamrei. The Lav and Aseh do not apply to one who transgresses a mid'Rabanan law, even b'Mezid, but does not deny the law.

Yad ha'Melech: We concluded that the Talmid erred about the Mitzvah to heed Divrei Chachamim. Rashi and the Rambam explain that he thought that even to do an Isur he must heed the Sanhedrin's Hora'ah. We suggested that a Talmid qualified to give Hora'ah can rely on himself that the Sanhedrin erred, and if he relied on their Hora'ah to transgress, he is Mezid, like one who intentionally transgresses. We concluded that he was Shogeg about the Mitzvah to heed Divrei Chachamim. This is unlike the Sifri, unless we say that the Sifri discusses one who thought that the Sanhedrin erred, but it was not clear to him. However, Rashi (Devarim 17:11) brings the Sifri simply, that even if he knows that they erred, he must follow them. The Mizrachi explains that even if they permitted eating Chelev, or killing based on one witness, he must follow them. This is unlike our Gemara! Why did they rule like the Sifri against our Gemara? The Rambam brought the verse "Lo Sasuru... right or left", but did not mention 'even if he tells you that right is left.' I do not know whether the Rambam rejected the Sifri due to our Gemara, or he omitted it for another reason. Perhaps he explains it like I resolved. The Yerushalmi brings a Hava Amina like the Sifri (to follow them even if they say that left is right), but expounds that we do not. The Rambam omitted it because it is unlike both Talmudim. Even though Rashi and the Mizrachi brought the Sifri simply, we rely on the Gemara and the Rambam.

Chidushei ha'Ran (Sanhedrin 89 DH R. Yehudah): If the Chacham is stringent about the matter that he forbids and the other Chachamim permit, the Ramban permits before he explained himself to them, but not after they heard his opinion and rejected it. Horayos and the Yerushalmi prove like this.

Heichal Yitzchak (EH 1:1:47): Why did the Gemara say that one who is coerced resolves due to the Mitzvah to heed Divrei Chachamim? It should have said that for a Mitzvah (which includes Mitzvos mid'Oraisa), he resolves! Rather, it teaches that even if he is a Chacham, and he thinks that the Beis Din errs (e.g. he need not divorce), it is a Mitzvah to heed the Beis Din. In Horayos we say that one may not heed the Sanhedrin if he knows that they erred (e.g. they permitted Chelev)! "Lo Sasur" merely forbids ruling unlike the Sanhedrin. It does not obligate him to eat Chelev! When Beis Din tells him to divorce, he must obey. He harms only himself, which is permitted, and helps his wife. (Magihah: Teshuvas ha'Rid says that for us, there is a Mitzvah to obey enactments of the Rosh Yeshivos. For us, the Yeshivos are like the Great Sanhedrin.

Yabi'a Omer (6 YD 7:2): The Terumas ha'Deshen (101) says that one should not be stringent due to one time that he transgressed. It is clear to him that one who never transgressed should not be stringent unlike the Gemara. Minchas Yakov (Soles Minchah 76:2) says that based on this and the Maharshal, it is like Apokorsus to be stringent about an Isur that became Batel in 60 or about a Keli Sheni, and one loses more than he gains. This is unlike Isur v'Heter (Sof 57), who says that even though one may not list more Tereifos than Chachamim did, one may be stringent not to eat a food in which Isur became Batel, or about a Keli Sheni, or not to eat meat after cheese in the same meal. We say that a Ba'al Nefesh should not eat from an animal about which a Chacham ruled! That is only when he ruled based on his own reasoning. The Rema (Sof 116) explicitly says so. Tzemach Tzedek (91) says that when Nosen Ta'am li'Fgam (if an Isur mixed with Heter harms the taste, it is permitted) depends on reasoning whether it harms the taste, e.g. ashes, we apply 'I did not eat from an animal that a Chacham ruled about' (it is proper to be stringent. When the law is clear) one may not be stringent on himself even to refrain. Chefetz Hash-m (Horayos 2b) asked from the Sifri, and answered that the Sanhedrin did not obligate one to eat Chelev of the stomach. They merely permitted it. Therefore, when it seems to him that it is forbidden, he should refrain. The Mitzvah to heed Divrei Chachamim is only when they command to do somethingl if one is passive and refrains, "Lo Sasur" applies. Pesach Einayim (Chulin 7) says similarly. However, in the Yerushalmi, Shmuel told Rav 'eat! If not, I will write that you are a Zaken Mamrei!' If this is because Rav publicized that he will not rely on the Heter of Rebbi's Beis Din for oil, how could the Rema bring a proof from it to his law? The Ramban (Sefer ha'Mitzvos Shoresh 1) says about the Sifri 'there is a condition for this. Study well the first Perek of Horayos (2b). I.e. if in the time of the Sanhedrin there is a Chacham qualified to give Hora'ah, and Beis Din permitted something and he thinks that they erred, he should not heed them and permit it to himself. He must be stringent, and all the more so if he is on the Sanhedrin. He should explain his reasons to them, and they will debate with him. If they refute him, he must accept their consensus.' The Ran learns from Horayos that before he told his reasons to them, he must be stringent. The Mitzvah to heed them is after he did so, and they rejected his opinion. Then, the Sifri says to follow them even if they say that left is right. This answers the question of Chafetz Hash-m. However, the Ramban (Devarim 17:11) says that the Sifri addresses one who thinks that they err, and it is as clear to him as the difference between his right and left hands. Do like they say. Do not say 'how can I eat absolute Chelev or kill this innocent person?!' Rather, say 'so Hash-m commanded me.' Chafetz Hash-m suggests that the Yerushalmi discusses one who is Gamir and Savir, and the Sifri discusses one who is not Gamir and Savir. However, if so, why is a Zaken Mamrei liable?

Note: Seemingly, one who is Gamir and Savir must be stringent for himself. A Zaken Mamrei is liable for ruling to others unlike the Sanhedrin!