Kesubos, Chapter 1, Mishnah 6. The Mishnah opens: "Hanosei es ha'ishah velo matza lah besulim" - a man married a woman assuming that she was a virgin, and after the marriage he claims that he did not find her to be a virgin. The woman herself concedes the fact, but the question under discussion is how the situation came about and when it happened.
The woman's claim:
"Hi omeres: mishe'irastani ne'enasti, venisteches sadecha" - after you betrothed me and I became halachically bound to you, but before we married and came to live together, I was raped, and as a result I ceased to be a virgin. "Nisteches sadecha," your field was flooded: the incident happened when I was already acquired to you and under your domain, and the responsibility does not rest on me. You betrothed me when I was a virgin, and this occurred afterwards, and therefore I am entitled to my kesubah.
The husband's claim:
"Vehu omer: lo, ela ad shelo irastich, vehayah mekach ta'us" - the husband claims that it happened before the betrothal, before she was bound to him at all, and it turns out that the acquisition of marriage by which he took her as a wife was a mistaken transaction, and he does not owe her the kesubah.
The dispute of the Tannaim:
"Rabban Gamliel veRabbi Eliezer omrim: ne'emenes" - in their opinion the woman is believed in her claim. The Gemara explains their reasoning on the basis of two principles:
A claim of certainty against a claim of doubt: Her claim is a claim of certainty - she was there herself and knows clearly what happened to her; the husband, on the other hand, has nothing more than a claim of doubt, that perhaps the matter occurred before the betrothal.
The presumption that she was a virgin: At the outset she had the status of a virgin, and on the strength of that presumption we leave the existing situation in place and attribute the change to the latest possible time, that is, after the betrothal.
It is the combination of these two principles that is decisive here, even though ordinarily one who comes to extract money from another's possession must bring proof. The rule is "hamotzi mechavero alav hare'ayah" - the one who makes the claim must bring the proof - and here the woman is seeking to collect her kesubah from the husband; nevertheless, because of the claim of certainty in her hands together with the presumption, she is believed according to Rabban Gamliel and Rabbi Eliezer.
The view of Rabbi Yehoshua:
"Rabbi Yehoshua omer: lo mipiha anu chayin, ela harei zu bechezkas be'ulah ad shelo tis'ares vehit'aso, ad shetavi re'ayah lidvareha" - we do not follow her words; rather, we establish her with the presumption that she was no longer a virgin already before the betrothal, and it turns out that she misled him. She does not collect her kesubah until she brings actual proof for her claim, that she was raped after the betrothal, at a time when she was already under his domain and the responsibility was not hers. In his opinion, the rule governing the extraction of money - "hamotzi mechavero alav hare'ayah" - overrides every other consideration.
In summary: Our Mishnah deals with a woman who was not found to be a virgin, where the two sides disagree about when it happened: she claims that she was raped after the betrothal and "his field was flooded," while he claims that it preceded the betrothal and his transaction was therefore a mistaken one. Rabban Gamliel and Rabbi Eliezer base her credibility on her claim of certainty as against the husband's claim of doubt, combined with the presumption that she was a virgin; Rabbi Yehoshua, however, holds that we do not follow her words, and on the strength of the rule "hamotzi mechavero alav hare'ayah" she does not collect her kesubah until she brings proof for her claim.