Ketubot 12 - What they did in Judea
In Judea people had a custom to celebrate the engagement with a big meal in the bride's father's house. Often the groom was left alone with the bride. This was done for him to become familiar with her, and also for another reason: ruling princes used to claim the right of the first night. If the bride was familiar with her groom, she would not submit willingly, and a woman who if violated is still permitted to her husband. However, because of the possibility of them having cohabited prior to the chuppah, in Judea the groom could not claim "not finding signs of virginity" in his wife, and thus could not deprive her of the full amount of the Ketubah obligation based on this claim.
So far, we never considered what the bride replies to the groom's claims. Let's look at this situation: the groom says that he did not find her a virgin. She explains that true, she was not, but that was because she was violated while being engaged to him, and thus he still owes her the amount of Ketubah if she becomes divorced or widowed, and this is a case of "your (buyer's) field became inundated," so it is his loss. He says that perhaps it is not so, but rather she was not a virgin when she got engaged to him, so his marriage to her was a "mistaken buy", and he does not owe the amount of Ketubah.
Rabban Gamliel says that she is believed. Why? Because her definite claim wins against his "perhaps," even though his argument is bolstered by the fact that the money is now in his possession - and she has the right to the complete Ketubah.
Art: Difficult Bride by Pavel Andreevich Fedotov