Ketubot 22 - I was married, but I am divorced now
A woman lived in the city, and everybody presumed that she was single. Then she makes a statement: "Actually, I was married, but I am divorced." She is believed, and she does not need to present the divorce letter, "Get." Why? Does not she render herself prohibited to the whole world with the first statement? - No, because we apply the principle of "the mouth that prohibits is the mouth that permits." In other words, we only know that she was married from her own words. Now that she says that she is divorced, we believe that also.
A similar example: a woman who says, "I was abducted by idolaters, but I am pure" is believed and can get married to a Kohen - even though normally any woman who had relations with an idolater cannot marry a Kohen any longer. If we only know about her capture from her, we believe her other statement. If, however, there was a witness to her being captured, then she must bring stronger proof.
Compare this to a case where a woman says that she was married, and then - that she was never married. Now she is not believed. Why not? Because unlike the first cases, her second statement contradicts the first. She must give a plausible explanation. For example, there was a very beautiful woman who used to say that she was engaged, but then she got engaged to one of her new suitors. She explained that previously unfit people used to woe her, and to get rid of them, she claimed that she was already engaged, and that now a proper person came. She was believed.
Art: The Abduction of Helen by Guido Reni