Masechet Shevuot, Perek Dalet, Mishnah Tet will continue to list cases in which somebody would be patur (exempt) for a violation of the oath concerning testimony: he claimed he did not know anything when, in reality, he had information that would have been helpful in somebody's legal situation.
The Mishnah says, “Mashbia ani aleichem.” Someone says to two people, “I am putting a swear on to you that when you become aware of a piece of testimony that would be beneficial to me when I'm involved in a disagreement about money in the future, you'll come and testify on my behalf.” The two people he addressed answer, “Amen” (so be it). They accept it and take that responsibility. If later on they do witness something, and this same person approaches them and tells them that he'd like them to testify on his behalf, then at that point they deny knowing anything. They make it seem as though they don't know what he was talking about or have any information that would be helpful to him in the legal situation. We say, once it becomes known that they lied, “Harei elu peturin.” They're not obligated to bring a korban (offering). Why? Because in this situation, the oath they took took place before they had knowledge of the testimony. The Torah tells us that witnesses would only bring a korban for something they could have testified about at the time they took the oath. Here, they swore. They didn't know anything. They didn't have any information that could have been helpful to him in the legal situation. And that testimony came up later on. So we say, not something that we're going to be, because the knowledge has to be there at the time—at the time of the swearing.