Occasionally, one will seek to appeal a Halachic decision that has been rendered. In this essay we will discuss the parameters of when Halacha permits one Rav to reverse the decision rendered by another Rav. In this context we will not discuss the institution of a Rabbinical Court of Appeals such as exists in Israel under the auspices of the Chief Rabbinate. For discussions of that institution see Rabbi J. David Bleich's Contemporary Halachic Problems IV: 17‑45 and Professor Eliav Schochetman's Seder Hadin 443‑470.
Gemara
The Gemara (Nidah 20b) relates a very interesting story that involved Yalta, a colorful Talmudic figure who was the wife of Rav Nachman. Yalta brought blood to Rabba Bar Chana for him to determine if she was a Nidah (see Tosafot s.v. Kol for a discussion of why she did not consult her husband, Rav Nachman, for a Halachic decision). Rabba Bar Chana ruled that she was a Nidah. Yalta, however, was dissatisfied with this decision and appealed the ruling to Rav Yitzchak the son of Rav Yehudah, who reversed the decision.
The Gemara is troubled by Rav Yitzchak's reversal because once a Rav has ruled that something is Tamei it is forbidden for another Rav to reverse that decision. The Gemara, in turn, explains that Rav Yitzchak at first told Yalta that she was a Nidah. Yalta, however, explained to Rav Yitzchak that Rabba Bar Chana had routinely ruled that the shade of blood that she showed him did not render her a Nidah. It seems, argued Yalta, that this time Rabba Bar Chana's eyesight was impaired and did not permit him to render an accurate decision. Rav Yitzchak accepted this argument as valid and ruled that Yalta was not a Nidah.
The Rishonim suggest several theories as to why Halacha imposes limits on the second Rav. Rashi (s.v. Meikara) indicates that the rule stems from concern for the dignity of the first Rav who was consulted. The Ran (to the Rif Avoda Zara 1b s.v. Hanishal) explains that reversal makes it "appear as if there are two Torahs." The Raavad (cited by the Ran) asserts that when one presents an issue to a Rav he binds himself to the jurisdiction of that Rav. It is equivalent to creating a prohibition by imposing a Neder (vow) upon himself. This is the Talmudic concept referred to as "Shavya Anafshei Chaticha D'issura."
Tosafot ‑ Nidah 20b and Avodah Zara 7a
Tosafot (Nidah 20b s.v. Agamrei) is troubled by Yalta's behavior in light of the Gemara's rule (Avodah Zara 7a) that if one posed a question to a Rav and he issued a strict ruling that he may not appeal the decision to another Rav. Tosafot explains that "the prohibition devolves on the Rav and not on the individual posing the question. The questioner may ask as much as he wishes ‑ as a result the Rav will be motivated to investigate the matter more thoroughly and sometimes as a result it will be discovered that the first Rav erred."
Tosafot in Avoda Zara (7a s.v. Hanishal) takes a more restrictive approach to this issue. He answers the question by explaining that the prohibition applies only when the questioner does not disclose to the second Rav that he previously presented the question to another Rav who issued a strict ruling. They also state that the second Rav is forbidden to reverse the first Rav's decision unless the former succeeds in convincing the latter that he erred. Tosafot in Nidah, by contrast, appears to permit the second Rav to reverse the first Rav's decision even absent the latter's consent.
Rama and Acharonim
The Rama (Y.D. 242:31) seems to offer a compromise between the two approaches of Tosafot. The Rama rules that the second Rav may overrule the first if the latter made a blatant error. This refers to a case where the first Rav erred in D'var Mishna, accepted Halachic practice (see Sanhedrin 33a, Rosh Sanhedrin 4:6, and Shulchan Aruch Choshen Mishpat 25:2). The Rama adds that if the second Rav believes that the original Rav made an error in judgment (what the Gemara calls "Taut B'shikul Hadaat") then the former should attempt to convince the latter to retract his ruling. However, if the first Rav refuses to retract his ruling, then the second Rav may not reverse the decision.
The Shach (Y.D. 242:53) cites differing opinions on the discussion of a greater Rav reversing the decision of a lesser Rav. The Shach appears to accept as normative the view that the great Rav is authorized to reverse the decision of a Rav of lesser stature. The Aruch Hashulchan (Y.D. 242:62) rules in accordance with the Shach. On the other hand, Rav Ovadia Yosef (Taharat Habayit 1:323) rules in accordance with the Rishonim who forbid even a great Rav to reverse the Halachic decisions of a Rav of a lesser stature. Needless to say, it is often exceedingly difficult to decide who is the Rav of greater stature.
The Aruch Hashulchan (Y.D.242:62) rules that if the original Rav was the /9! $!;9! (Rav of the area or synagogue) then under no circumstances may his decisions be reversed.
Rav Ovadia Yosef writes (Taharat Habayit 1:331) that if a Rav rendered a Halachic decision for a Sephardic Jew in accordance with Ashkenazic authorities but conflicting with traditionally accepted Sephardic authorities such as Rav Yosef Karo, the decision may be overturned because Rav Yosef Karo is viewed as the /9! $!;9! of Sephardic Jews.
Accordingly, the question of when a Rav may reverse a decision of a colleague is a complex issue. For a thorough review of this issue see Encyclopedia Talmudit 8:507‑510.