Commentary for Sanhedrin 3:8
כָּל זְמַן שֶׁמֵּבִיא רְאָיָה, סוֹתֵר אֶת הַדִּין. אָמְרוּ לוֹ, כָּל רְאָיוֹת שֶׁיֶּשׁ לְךָ הָבֵא מִכָּאן עַד שְׁלֹשִׁים יוֹם. מָצָא בְתוֹךְ שְׁלֹשִׁים יוֹם, סוֹתֵר. לְאַחַר שְׁלֹשִׁים יוֹם, אֵינוֹ סוֹתֵר. אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל, מַה יַּעֲשֶׂה זֶה שֶׁלֹּא מָצָא בְתוֹךְ שְׁלֹשִׁים וּמָצָא לְאַחַר שְׁלֹשִׁים. אָמְרוּ לוֹ הָבֵא עֵדִים וְאָמַר אֵין לִי עֵדִים, אָמְרוּ הָבֵא רְאָיָה וְאָמַר אֵין לִי רְאָיָה, וּלְאַחַר זְמָן הֵבִיא רְאָיָה וּמָצָא עֵדִים, הֲרֵי זֶה אֵינוֹ כְלוּם. אָמַר רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל, מַה יַּעֲשֶׂה זֶה שֶׁלֹּא הָיָה יוֹדֵעַ שֶׁיֶּשׁ לוֹ עֵדִים וּמָצָא עֵדִים, לֹא הָיָה יוֹדֵעַ שֶׁיֶּשׁ לוֹ רְאָיָה וּמָצָא רְאָיָה. אָמְרוּ לוֹ הָבֵא עֵדִים, אָמַר אֵין לִי עֵדִים, הָבֵא רְאָיָה וְאָמַר אֵין לִי רְאָיָה, רָאָה שֶׁמִּתְחַיֵּב בַּדִּין וְאָמַר קִרְבוּ פְּלוֹנִי וּפְלוֹנִי וְהַעִידוּנִי, אוֹ שֶׁהוֹצִיא רְאָיָה מִתּוֹךְ אֲפֻנְדָּתוֹ, הֲרֵי זֶה אֵינוֹ כְלוּם:
Whenever he brings proof (in his favor), he can overturn the verdict. If they said to him: "Whatever proofs you have, bring them within thirty days" — If he found (proof) within thirty days, he overturns it; if not, not. R. Shimon b. Gamliel asked: "What is one to do if he did not find it within thirty days but did find it afterwards!" If they (beth-din) said to him: "Bring witnesses," and he said: "I have no witnesses"; if they said: "Bring proof" [a writ of credit], and he said: "I have no proof," and afterwards he brought proof or found witnesses, it is of no account. [For he said "I do not have," and we suspect him of forgery or of having hired false witnesses.] R. Shimon b. Gamliel said: "What is one to do if he did not know that he had witnesses and he found witnesses, or if he did not know that he had proof and he found proof!" [The halachah is not in accordance with R. Shimon b. Gamliel.] If they said to him: "Bring witnesses," and he said: "I have no witnesses"; "Bring proof," and he said: "I have no proof," and then, seeing that he would lose the case, said: "You and you come here and testify for me," or he produced proof from his afundah [(his belt; others say: a garment worn close to the skin)], it is of no account. [In this, even R. Shimon b. Gamliel concurs. For since he knew of them and denied it, he is certainly a liar. But if one says: I have witnesses or proof across the seas, he is not heeded to delay judgment until he sends abroad; but the ruling is given according to the evidence available at that time, and when he brings witnesses or proof, the ruling is overturned and the case is heard again as per the witnesses or proof that he brought.]
Bartenura on Mishnah Sanhedrin
English Explanation of Mishnah Sanhedrin
If they had said to him, “Bring all of the proofs that you have within thirty days” and he brought them within thirty days, the court may overturn the verdict. But if he brought any proof after thirty days, the court cannot reverse the verdict. Rabban Shimon ben Gamaliel said: “What could he have done that he did not find [the proof] within thirty days but found it after thirty days?”
If they had said to him, “Bring witnesses” and he said, “I have no witnesses”, or [if they said], “Bring proof”, and he said, “I have no proof”, and he later found proof or witnesses, then they are totally invalid. Rabban Shimon ben Gamaliel said: “What could he have done that he did not know that he had witnesses, then found witnesses, or that he did not know that he had proof, then found proof?
If they had said to him, “Bring witnesses” and he said, “I have no witnesses”, or [if they said], “Bring proof”, and he said, “I have no proof”, but when he saw that he was about to be found obligated, he said, “Come near, so-and-so and so-and-so and testify for me!”, or if he brought forth some proof from his wallet, then they are totally invalid.
Mishnah eight discusses the ability of the litigants to overturn the verdict by bringing new evidence (physical proof or witnesses) after the trial.
Mishnah eight discusses the situation where a person did not bring proof or witnesses until after he was found to be obligated. If the court had not told him that he had a time limit on bringing such evidence, he may always bring it and thereby give the judges reason to overturn the verdict. If, however, the court had set a time limit on his bringing proof or witnesses, and that time limit elapsed, he may no longer attempt to overturn the verdict. Since the court set a time limit, anything brought afterwards is not considered relevant to the trial. Rabban Shimon ben Gamaliel disagrees with this time limit. According to him even if a time limit was set, if the defendant should find new evidence he may bring it to the court’s attention and have the decision overturned. In other words there is a dispute between Rabban Shimon ben Gamaliel and the Sages (the anonymous opinion), the former holding that time limits are irrelevant and the latter that they are valid.
The final clause of the mishnah contains an opinion to which even Rabban Shimon ben Gamaliel agrees. Even though he generally allows evidence to be brought later, if it is clear that the defendant could have known about the evidence and chose not to use it and then produced it just at the moment when the verdict was about to be delivered against him, the evidence is invalid. In such a case we may rightfully wonder why he didn’t bring the evidence earlier and we suspect him of lying.