Witnesses can be refuted and become false witnesses in the following way: a second set of witnesses comes and testifies that the first witnesses could not have possibly seen the incident to which they testified, since they were with them (the second set) in a different place at that very time. Refuted false witnesses get the same punishment as what they wanted to inflict on the one about whom they testified.
If witnesses testified in January, but later were exposed as false witnesses in July, then what happens to their unrelated testimony given in March? Abaye says that they are disqualified retroactively, since they are evildoers. Rava says that the law of false witnesses is such a novelty that it cannot be analyzed logically; they are false only from now on, and their March testimony is unaffected.
This is the "A" in the six cases abbreviated Y A L K G M where the law follows Abaye against Rava.
If a witness eats non-kosher meat because it is cheaper, then he cannot testify, because he can be bribed. What if he eats it to make a point? Abaye says that he is an evildover, thus disqualified. Rava says that for disqualification he needs to be an evildoer in matters of money, not ritual.
Art: Thomas Sully - Gossip