1) I didn't understand what the Gemara meant in discussing the "shevuas ha'edus," case where 2 witnesses said: "You knew testimony," and he said: " I didn't." He's patur from a korban since he could say: "I didn't have kavanna to give edus." (I saw Rashi and the shita mekubetzes.) My question is that in shevuas ha'edus he swore in Bais Din that he didn't know anything about this case..a loan..a pikadon..whatever it was & he really did know aydus. How can he say I didn't have kavanna to give aydus. Never mind what he had in mind when he saw the original act. He swore in Bais Din that he didn't know anything. That's not an unconscious act. They probably warned him of the consequences of swearing falsely. (Whoever swears falsely it's as if he destroyed the world ...)
2) Also, correct me if I'm wrong but doesn't he have to admit his lie on his own in such a case in order to bring a korban asham (it's an oleh v'yored, I think.) The Torah wants him to come in on his own and admit that he lied. Wouldn't two eydim not make him liable to bring a korbon asham? So what kind of migu, (or whatever this tayna is in the place of witnesses), can this be?
1) A person may be causing immeasurable damage to the world by swearing falsely, but he is not obligated to bring a Korban of Shevu'as ha'Edus except under specific circumstances. If a witness cannot testify in court to the financial benefit of his friend, for whatever reason it may be (he is Pasul l'Edus, he did not have intent to testify when watching a monetary transaction and therefore forgot the exact details, he was not appointed as a witness when the Ba'al Din was Modeh, etc.), then the witness is not obligated to bring a Korban.
2) The Pasuk of v'Hisvadah (quoted in the Beraisa at the end of 12a), about which we are told "Edim are no less than personal admission," applies to all those who bring an Oleh v'Yored, including Shevu'as ha'Edus.