Gemara

2a (Mishnah - Shamai): If any woman sees that she became a Nidah, she is Teme'ah from now and onwards, but not retroactively;

Hillel says, she is retroactively Teme'ah from her last Bedikah (when she checked herself).

Question: What is Shamai's reason?

3a - Answer #1: Shamai holds that a woman is (usually) Margish (senses when Dam leaves the Makor. Since she was not Margish before, we assume that it came now.

Shamai admits that a lunatic is retroactively Teme'ah.

Answer #2: Shamai holds that had the blood entered the Beis ha'Chitzon earlier, it would have left her body right away. The walls of the Rechem (womb) do not hold in blood.

3b - Question: What is the difference between these answers?

Answer: Answer #2 is retroactively Metamei one who uses a wad that fills the width of the Rechem and stops blood from exuding. Answer #3 is Metaher.

Kidushin 66b (Beraisa - R. Tarfon): If a Mikveh was once known to be full, and it was found to be deficient, we are not Metamei anything Tamei that was immersed in the Mikveh.

R. Akiva says, it is Tamei.

R. Tarfon: The Chazakah is that the Mikveh was full. If there is a doubt about its status at the time that people or Kelim immersed, don't assume that it was deficient! This is like a Kohen who was offering Korbanos, and it became known that he was a Chalal (born to a divorcee). What he offered is acceptable.

R. Akiva: No, it is like a Kohen who was offering, and it became known that he was blemished. What he offered is not acceptable. One witness can testify that a Mikveh is deficient or that a Kohen is blemished, but two witnesses are needed to testify that a Kohen is a Chalal;

Also, a deficient Mikveh and a blemished Kohen are disqualified due to themselves, unlike a Chalal, who is disqualified due to others (his mother).

R. Tarfon agreed to R. Akiva.

Sanhedrin 25a: Ploni testified 'I saw Bar Binisus borrow on Ribis', and Almoni testified 'I borrowed from Bar Binisus on Ribis.' Rava disqualified Bar Binisus.

Question: Rava said that one who borrows on Ribis is disqualified. If so, Almoni is a Rasha - "Al Tesht Rasha Ed"!

Answer: Rava ruled like another of his teachings;

(Rava): One is considered a relative with respect to himself, so he cannot disqualify himself through saying that he is a Rasha.

26b (R. Avahu): All of the Pesulim (for testimony) in our Mishnah must be announced by Beis Din. (Until this they are Kosher.)

Two thieves were signed on a gift document. Rav Papa bar Shmuel thought to honor the document, because no announcement was made about them.

(Rava): No. Announcement is only for thieves mid'Rabanan. Thieves mid'Oraisa do not need announcement.

Rishonim

Rambam (Hilchos Edus 11:6): If a witness is disqualified mid'Oraisa, his testimony is invalid even if they did not announce about him in Batei Kenesiyos and Batei Medrash. If a witness is disqualified mid'Rabanan, they must announce about him. Therefore, whatever testimony he gave before they announced, we accept it, lest people lose their Zechus, for they did not know that he is Pasul, and his Pesul is only mid'Rabanan.

Nimukei Yosef (Sanhedrin 4b DH v'Havah): One who borrowed with Ribis is Pasul for testimony. Even though a Pesul mid'Rabanan is not disqualified until there is announcement, we require announcement only to prevent loss to buyers. Letter of the law, he is disqualified (immediately). Here (the borrower testified to disqualify the lender), there is no loss to buyers, so (the Gemara asked that) he should be Pasul (immediately). The Ro'oh and others answered that announcement is needed to disqualify him only for other testimonies, but for the testimony that disqualified him, Chachamim did not believe him, even without announcement.

Rivash (266, cited in Beis Yosef CM 34 DH Kasav ha'Rivash): I do not see any proof that Ploni transgressed the Cherem before he testified about the Kidushin. They testified Stam that Ploni transgressed; they did not say when. Therefore, he is disqualified only from when they testified. We leave him on his Chazakah until then.

Poskim

Shulchan Aruch (CM 34:23): If a witness is disqualified mid'Oraisa, his testimony is invalid even if they did not announce about him in Batei Kenesiyos and Batei Medrash. If a witness is disqualified mid'Rabanan, testimony he gave before they announced is Kosher.

Rema: Even if someone is Pasul mid'Oraisa, we disqualify him only Vadai, but not due to Safek. E.g. if someone testified, and later witnesses said that he transgressed an Aveirah that disqualifies him mid'Oraisa, but they are unsure whether this was before or after he testified, we leave him on his Chazakah, and everything he testified is Kosher.

SMA (56): For the testimony due to which he was disqualified, the Nimukei Yosef disqualifies him even without announcement.

Rebuttal (Shach 23): This is difficult. The Nimukei Yosef said so only according to the questioner, who thought that his own words disqualified him for testimony. If so, when he says 'he lent to me with Ribis', he is not believed, for he is disqualified. However, we answer that a person cannot make himself a Rasha (disqualify himself) and we divide his words, so this law does not apply. If others testify that he borrowed with Ribis, this is another testimony. Why did the Beis Yosef and SMA bring it? This requires investigation.

Shev Shematsa (3:4): The Beis Yosef (Bedek ha'Bayis, YD Sof Siman 119) brings from Orchos Chayim, citing Tosfos, that if Ploni was Memir Daito (became an idolater), he is not suspected retroactively about Shechitah. Tevu'os Shor (2:32) and the Bach (119) are stringent. However, the Shach (1:8) and Pri Chodosh (1:9) do not forbid retroactively. The Maharshal (Chulin 1:7) brought a Teshuvah to be lenient, but did not explain the reason.

Shev Shematsa (DH v'Achar): Really, it seems that the Shach is correct. We hold that an Edim Zomem is Pasul retroactively, i.e. from when he testified falsely (Bava Kama 72b). What he testified before that is Kosher. Rabanan and R. Yosi argue about witnesses who testified about the theft, and afterwards (Toch Kedei Dibur, within the time to say three or four words) about the Shechitah. Rabanan hold that Toch Kedei Dibur is unlike Dibur (they are like two separate testimonies). If they were Huzmu about the Shechitah, they were not Huzmu about the theft. If we disqualify Shechitah because we say that he steadily was becoming more evil, so we establish the animal in its Chezkas Isur, just like a Mikveh found to be deficient, all the more so if they testified about money, we should leave the money in the Chazakah of its original owner. Chezkas Mamon is the strongest Chazakah. This is clear from the Sugya of Mumim found in a Kalah. See what I wrote above (2:9). It is clear from the Gemara that his testimony about money is Kosher before he testified falsely, and all the more so what he testified about Isur. This is clear from the Tur and Shulchan Aruch CM 34:8.

Shev Shematsa: Another proof is from Shechitah on Shabbos. (It is Kosher.) Tosfos (Chulin 14a DH ha'Shochet) establishes this when he was Mezid. Shechitah of a Mechalel Shabbos is Pasul! Tosfos says that he is not established for this Shechitah. The Rambam says that he is not established to be Mechalel Shabbos until he slaughters the majority. This shows that he is not disqualified retroactively. I say that Maharshal is lenient because this is unlike a Mikveh. Rashi (Kidushin 66b DH Mikveh) explains that R. Akiva does not learn from a Kohen found to be a Chalal (his Avodah is Kosher b'Di'eved), because we need two witnesses to establish him to be a Chalal. He learns from a Kohen found to be a Ba'al Mum, for one witness (who can show the Mum) establishes this, just like a Mikveh. The Pesul of someone suspected is through two witnesses, therefore it is not retroactive. If one did not know the laws, the Taz (1:6) forbids retroactively only because there also, one can say 'I will show you his mistake (that he does not know the laws).' Tosfos (66b DH Amar) say that a Mikveh is unlike a Chalal. If so, why isn't someone suspected disqualified retroactively? Regarding money, Chezkas Mamon supports the current Pesul of the witness. Regarding a butcher, Chezkas Isur of the animal supports his Pesul. We disqualify retroactively only in cases like a Mikveh found to be deficient, a knife found to be nicked, or if one slaughtered and was found not to know the laws. In all case we do not know when this happened. However, if a Shochet was Memir Daito, we have no Safek that perhaps he did so also earlier, since there is no doubt about when he did the Aveirah we know about.

Tosfos (3b DH l'Abaye): The Gemara could have said that they argue about a lunatic. If Shamai holds that she is not Temei'ah retroactively because she feels the blood, Shamai agrees about a lunatic. Do not say that 'if blood was there, it would have come earlier' teaches that Shamai agrees about a lunatic (according to that reason), for perhaps other blood fell out before the Bedikah. Rather, we discuss the blood found now.

Shev Shematsa: We learn from Tosfos that even Hillel, who is Metamei back to her last Bedikah, and is stringent (about burning) Kodshim and Terumah for when the blood was found, is not concerned for other blood. The only concern is lest the blood found was already in the Beis ha'Chitzon earlier. Similarly, if one became a Rasha now, we have no Safek about another evil act. Therefore, his previous testimony is Kosher, even for money against a Chazakah. However, based on this, if witness testified that Ploni did an Aveirah that disqualifies him for Shechitah or (testimony about) money, and we do not know whether he transgressed long ago or today, this is like a Mikveh, since we are unsure about the episode itself. Chezkas Mamon or Chezkas Isur joins with his current Pesul (he is a Rasha). Why does the Rema leave the witness on his Chazakah, and not disqualify due to Safek? This is like the Rivash. Why is this unlike a Mikveh? The Rif (Bava Kama 28b) supports the Rivash. If witness signed falsely on a document, we disqualify them from then only if witness saw them sign. If not, we say that they signed now. Perhaps the Rif holds like Rashi, that R. Akiva does not learn (to be Metaher retroactively) a Mikveh from a Chalal, for two witnesses are needed to disqualify a Chalal. If so, we do not disqualify a witness retroactively, for Pesul requires two witnesses. However, according to Tosfos, if we are unsure when he sinned, we leave money in its Chazakah, for there are two Re'usos (reasons to be concerned).