Bechoros chapter 4, Mishnah 4. The Mishnah begins: "Mi she'eino mumcheh vera'ah es habechor venishchat al piv - harei zeh yikaver" - one who is not an expert and examines a firstborn, and it is slaughtered based on his word, it must be buried. A person who is not authorized as an expert, even if he considers himself knowledgeable in the laws of blemishes, and even if he is the greatest of the generation, as long as he did not receive proper authorization, he is not an expert from a halachic perspective. If such a person examined the firstborn, ruled that it is a permanent blemish and permitted it to be eaten, and based on his instruction the firstborn was slaughtered, we do not eat it but rather bury it. This is because the halachah follows Rabbi Meir in the previous Mishnah: if one did not receive appropriate permission prior to the slaughter, the firstborn may not be eaten based on his word. The result is that the kohen lost his firstborn.
And what is the halachah regarding that decisor who ruled without permission, since the blame lies with him? The Mishnah states: "Viyshalem mibeiso" - he must pay from his own pocket and compensate the kohen who lost his firstborn. Even if the firstborn truly had a blemish, since the kohen cannot eat it, he was caused a loss and must be compensated.
The amount of payment:
The Gemara discusses the question of how much he must pay, and differentiates between a large animal and a small animal:
Large animal (cow): He pays fifty percent of the value of the carcass. The reason: there is no way to know what a true expert would have ruled. He might have determined that this is not a permanent blemish, and then the animal would have died in its unblemished state and would never have been eaten. The result is an evenly balanced doubt as to whether the kohen actually suffered a loss, and therefore he pays half. This amount is fixed, and we do not weigh probabilities or distinguish between a severe blemish and a light one.
Small animal (goat or sheep): He pays twenty-five percent. First, due to that same evenly balanced doubt; and additionally, even if the animal did not have a blemish, there is trouble and expense in pasturing goats and sheep. That decisor, who permitted its slaughter without authorization and caused its owner a loss, saved him the trouble of pasturing in the event that the expert would have forbidden its slaughter, and therefore we divide the amount in two again.
It is worth noting that the Shulchan Aruch rules that this halachah was stated specifically in Eretz Yisrael, because in Eretz Yisrael there is a separate rabbinic prohibition against raising small animals, goats and sheep, in settled areas. The reason is simple: a person does not want his neighbor's goats to eat his garden, and this is the way of goats. Therefore one must take the goats out to the desert and to unsettled areas, and there is trouble and expense involved in the matter, and the result is that the decisor saved the owner a lot of money. For this reason the Shulchan Aruch rules that in Eretz Yisrael the payment is a quarter, while outside the Land it is a half, because that rabbinic enactment (which was designed to protect the settlement of Eretz Yisrael and the pleasantness of life there, and for this reason we do not raise goats in a settled area) does not apply outside Eretz Yisrael, and consequently the obligation reverts to fifty percent.
A judge who erred in his judgment:
Here the Mishnah begins a new paragraph (and in the Babylonian Talmud this is a completely separate Mishnah), and transitions from the laws of firstborns to the general principle: a judge or a rabbi who issued a halachic ruling and through his error caused a loss to the litigant, must he pay? The language of the Mishnah: "Dan es hadin, zikah es hachayav vechiyav es hazakai, timei es hatehor vetihar es hatamei" - whether he judged a case and acquitted the guilty and convicted the innocent, or declared the pure impure and declared the impure pure, whether in monetary laws where he acquitted the guilty and convicted the innocent, or in matters of prohibition and permission, such as a kohen who asked about his Terumah and he ruled that it is impure and he is not permitted to eat it, or he permitted him to eat it and the kohen mixed it with the rest of his food and everything became impure, because in truth it was impure.
The rule is: "Mah she'asah asuy" - the ruling that the judge issued stands and we do not nullify it; "viyshalem mibeiso" - but if he caused a loss by doing so, he must compensate the damaged party from his own pocket.
At the root of the matter are two types of errors: an error in judgment and an error in a clear Mishnah. An error in judgment means that the judge weighed all the data and reached a conclusion, and the next day realized that he had erred and did not take a certain piece of data into account, and he wishes to change his ruling. The rule is that he cannot change it, because the one who won the case yesterday will argue: today you say no, and tomorrow will you say yes again? Therefore his ruling remains intact and we do not nullify his words - "mah she'asah asuy". However, "viyshalem mibeiso": according to this Tanna, since the judge caused a loss to a litigant who should not have had to pay, or who was supposed to receive what was his and did not receive it, he is considered damaged and the judge is responsible to compensate him.
The Mishnah continues: "Ve'im hayah mumcheh leveis din - patur milshalem" - and if he was an expert for the court, he is exempt from paying. All of the above was said regarding a judge who was not authorized by the court, the Sanhedrin, even though he is recognized as knowledgeable; but one whom the Sanhedrin recognized as an expert and who is authorized to serve as a judge is immune from such a loss and is exempt from paying.
An incident with Rabbi Tarfon:
The end of the Mishnah brings an incident that illustrates this principle. We will briefly introduce the story, as its structure in the Mishnah is slightly complex: Rabbi Tarfon ruled that a cow whose womb was removed has the status of a treifah, and it may not be eaten but must be given to the dogs. The Sages heard his ruling and determined that it was factually incorrect. Todos the physician, a famous doctor in the Mishnaic period, reported in another context that every cow and sow (female pig) that leaves Alexandria in Egypt has its womb removed first so that it cannot produce offspring, thereby allowing the people of Alexandria to maintain their monopoly in the market for fine cows and pigs. This proves that a cow whose womb was removed is not going to die and is not a treifah, which meant that Rabbi Tarfon's ruling was mistaken.
Meanwhile, Rabbi Tarfon had already instructed the owner of the cow that it was not kosher and had to be given to the dogs, thereby causing him a financial loss. Rabbi Tarfon exclaimed: Your donkey is gone, Tarfon - now you must sell your donkey to compensate the victim. Rabbi Akiva responded and told him that he was mistaken in thinking he had to pay: Although he erred in his ruling, he did not have to pay compensation, since he made an honest mistake. The rule is that someone who has been recognized by a beis din as an expert and authorized to serve as a judge is protected from such an unintentional error.
And the text of the Mishnah itself:
"Maaseh beparah shenitlah ha'eim shelah" - "eim" here means not only a mother but a womb, which was removed from the cow. "Vehe'echilah Rabbi Tarfon lakelavim" - he instructed that it be given to the dogs, meaning he ruled that it was forbidden for human consumption because it was a treifah. "Uva maaseh lifnei chachamim vehitiru" - the Sages heard about this ruling and determined that a cow whose womb was removed is kosher and is not a treifah. "Amar Todos harofeh: Ein parah vachazirah yotzah me'Aleksandriyah ad shehem chotchin es ha'eim shelah bishvil shelo teiled" - this is the proof for this matter.
"Amar Rabbi Tarfon: Halchah chamorecha Tarfon" - meaning, say goodbye to your donkey, since you must sell it to compensate the person who received your mistaken ruling. "Amar lo Rabbi Akiva: Rabbi Tarfon, patur atah, she'atah mumcheh leveis din" - you have already been recognized by the Sanhedrin as an expert and authorized to serve as a judge, and therefore you do not have to compensate the harmed party. "Vechol mumcheh leveis din patur mileshalem".
And such is the halachah: An expert recognized as such by a beis din is exempt from paying. In practice, the Rema rules that there is no need for a formal authorization from the Sanhedrin. Rather, any ordained rabbi or judge who acted properly is exempt, because otherwise, there would be no judges willing to issue rulings, and the world cannot function this way. Thus, a judge who makes an honest mistake is not obligated to compensate the party that suffered a loss due to his inadvertent error.