Sanhedrin 27A

עד זומם, אביי אמר: למפרע הוא נפסל; ורבא אמר: מיכן ולהבא הוא נפסל.

concerning a conspiring witness: Abaye says: He is disqualified retroactively, from when he provided his testimony. Any testimony he may have provided after that point in time is retroactively nullified. And Rava says: He is disqualified only from that point forward, i.e., from when he was established to be a conspiring witness, but not retroactively from when he provided his testimony.

אביי אמר: למפרע הוא נפסל, מעידנא דאסהיד רשע הוא, והתורה אמרה: ״אל תשת ידך עם רשע״⁠ – אל תשת רשע עד.

The Talmud explains the reasons for the two opinions: Abaye says he is disqualified retroactively because it is from that time when he testified that he is considered a wicked man. And the Torah stated: “Do not put your hand with the wicked to be an unrighteous witness” (Exodus 23:1), which is interpreted to mean: Do not allow a wicked man to serve as a witness.

רבא אמר: מיכן ולהבא הוא נפסל. עד זומם חידוש הוא, מאי חזית דסמכת אהני? סמוך אהני! אין לך בו אלא משעת חידושו ואילך.

Rava says that he is disqualified only from that point forward because the disqualification of a conspiring witness is a novelty, i.e., it is not based on logic. The reason is that this is a case of two witnesses against two other witnesses, in which case the testimony of neither should be accepted. What did you see that causes you to rely on the second set of witnesses, who testify that the first set were not at the scene of the purported event? You could instead rely upon the first set of witnesses, who testify to the event, and consequently disbelieve the second set. Yet the Torah teaches that the second set of witnesses is always deemed credible and the first set is subjected to punishment as conspiring witnesses. Therefore, as the disqualification of a conspiring witness is an anomaly, you have the right to disqualify him only from the time of its novelty and onward; this counterintuitive disqualification is not applied retroactively.