"With your permision, Mother," Siuan said in a husky voice, "I'll send Elin to fetch the Keeper's serving woman to do what's needful."

"Stay!" Tamra barked. That iron-hard gaze studied them both. "You will tell no one about this, not for any reason. If necessary, lie. Even to a sister. Gitara died without speaking. Do you understand me?"

As it's rather obvious to Moiraine and Siuan that Gitara did say something before she died (the prophecy that the Dragon had been reborn at that very moment), many fans have wondered how the Amyrlin was able to say something untrue. Shouldn't the Three Oaths prevent her from lying?

Only if Tamra intended for the Accepted to believe her. The Oaths are triggered by intent, not absolutes. Tamra knows full well that Moiraine and Siuan won't believe her when she says "Gitara died without speaking." She's stating it as part of her directive to them to concel this Foretelling at all costs, even to the point of lying themselves.  Aes Sedai can say something untrue if A) they believe it to be true or B) it's not intended to be believed.

Jordan clarified this himself in a blog entry dated January 21st, 2006:

...the oath against lying does leave room for sarcasm. It is intent and result that matter. No sister can intentionally speak an untruth either with the intent of passing on false information or with the belief that false information might be passed on. Thus the careful slicing and dicing of words. But if someone were to hold up a piece of white cloth and ask whether it was black or white, someone who had sworn the Three Oaths would be capable of saying that it was black as a matter of sarcasm. But not if, for example, the person asking the question was blind and thus might well take the statement for truth rather than sarcasm.

Besides, if Tamra was able to lie, that would make her a member of the Black Ajah, the only Aes Sedai we've seen with the ability to lie. And if she was Black Ajah, then there would have been no need for the Black to torture her to obtain information about the Dragon's rebirth.  The current Head of the Black could have just ordered her to reveal the Foretelling and the series would have been much, much shorter.