Sanhedrin, Chapter 3, Mishnah 5. At the end of the previous Mishnah, we discussed the opinion of Rabbi Yehudah regarding a situation where a person is disqualified from testifying even though he is no longer considered a family member, and we do not wish to go back into that now. Rabbi Yehudah continues his statement at the beginning of our Mishnah as well: the Tanna Kamma before us, whose name is not mentioned, is Rabbi Yehudah, and he establishes that "ha'ohev vehasonei" - both a close friend and a bitter enemy - are also disqualified from testifying.
Who are the friend and the enemy in the Mishnah:
"Ohev - zeh shoshvino" - this refers to a groomsman. Those who study the tractates in order saw the laws of the groomsman at the end of Bava Basra: these are extremely close friends, who even assist with the wedding expenses. In particular, this refers here to the week of Sheva Berachos, during which the groomsmen are considered especially close and concern themselves with the groom's welfare.
"Sonei - kol shelo dibber immo sheloshah yamim be'eivah" - a person who is so angry with his fellow that he refrains from speaking with him for three days, out of anger and enmity, and refuses to converse with him.
In the opinion of Rabbi Yehudah, these people are biased by their love or hatred toward that person to the point that there is concern they will not tell the truth in their testimony.
The Sages reject this: "Amru lo: lo nechshedu Yisrael al kach" - a Jew who is not known to be wicked or immoral is not suspected of such a thing. For there is almost nothing more severe than delivering formal testimony and lying about it, and a person will not offer false testimony, destroy his integrity, and commit such a terrible act, merely because he loves the litigant or hates him. This is the dispute.
The distinction between a judge and a witness:
First, it must be clarified that our Mishnah deals with testimony. However, when it comes to serving as a judge in such a situation, where the judge is the close friend or bitter enemy of one of the litigants, everyone agrees that this person is disqualified from judging. At lesser degrees of affection or hatred, many maintain that it is merely a measure of piety that he should not judge, but ideally it is certainly proper that he not serve as a judge, so that no feeling arises that the trial is unfair and that things are not as they should be.
And what is the law regarding a judge who is not technically disqualified, whose conflict with the litigant is not bitter to the point of refraining from speaking with him for three days, and yet he has excessive closeness or aversion toward one of the litigants, and nevertheless he judges? The Shulchan Aruch rules that after the fact he is valid and the judgment stands. The Rema rules that the judgment does not stand, since it is inconceivable to have a situation where one of the judges leans strongly for or against one of the litigants, and therefore his ruling is not binding.
The halachah:
The prevailing rule is that for anyone who is disqualified from serving as a judge, the same rules apply to him regarding his fitness for testimony. And indeed, the Bartenura writes that the halachah follows Rabbi Yehudah, that the friend and the enemy are disqualified from testifying. In practice, however, this is not the halachah, despite the words of the Bartenura, since the ruling follows the Sages: a Jew is not suspected of lacking morality and integrity to the point that he would lie on account of his relationship with the litigants.
And it must be remembered that this situation is extremely common. Let us consider an example: Reuven lends money to Shimon and wishes to bring witnesses. Who will his witnesses be? Presumably his close friends. According to the logic of Rabbi Yehudah, the friends would not be able to testify, on account of their excessive closeness and the concern that they would lie. But the halachah is established that the friend and the enemy are disqualified from serving as judges, whereas for testimony they are valid. This is therefore an exception to the rule: usually, those disqualified as judges and those disqualified as witnesses go hand in hand, and here they diverge.
Rabbi Yehudah's source:
There is a precedent in the Torah for suspecting someone of being a bitter enemy, and it appears in the context of an unintentional murderer, a case where a person may be sent into exile. In this section, two verses are stated:
"V'hu lo oyeiv lo v'lo mevakeish ra'aso" - it is inconceivable that the person murdered unintentionally unless he did not harbor that known hatred toward the one he supposedly "murdered" by mistake. From here we see the concern that a person might be driven to murder out of hatred.
"V'hu lo sonei lo mit'mol shilshom" - this is the second mention of the same concept of bitter hatred, and it refers to someone who has not spoken with that person in the last three days: "mit'mol shilshom", meaning yesterday and the day before.
Based on these two verses, Rabbi Yehudah derives that the matter applies even to the cases in our Mishnah, and this is the precedent. Just as a witness is liable to be biased due to his hatred, so too is he liable to be biased due to his close friendship, at least during the week of Sheva Berachos, which is a unique situation where a person feels an extra inclination to "help" his friend. But the Sages reject this in all cases: these concepts apply only to judges, but not to witnesses.
In summary: In this Mishnah, Rabbi Yehudah and the Sages disagree regarding the law of a friend (the groomsman) and an enemy (who has not spoken with him for three days out of enmity) concerning testimony. Rabbi Yehudah disqualifies them due to the concern of favoritism, deriving this from the verses in the section of the unintentional murderer, whereas the Sages respond, "Jews are not suspected of this." The halachah follows the Sages, and therefore these individuals are valid for testimony but disqualified from judging, and this is an exception to the rule that those disqualified from judging and those disqualified from testifying are identical.