Kesubos, Chapter 1, Mishnah 4. This Mishnah deals with women who, although they are presumed to be virgins, receive a kesubah of only a maneh rather than two hundred, and against whom the husband has no claim regarding virginity.
The first part of the Mishnah:
"Besulah almanah, gerushah vachalutzah min hanisuin - kesubasan maneh" - this refers to a woman who is presumed to be a virgin, and nevertheless is a widow, a divorcee, or one who received chalitzah, and all of this came about through marriage: she was actually married to a man, and afterward he died, or she was divorced from him, or he died and afterward she received chalitzah. The kesubah she receives when she remarries is only a maneh - the sum fixed for a widow and not for a virgin.
Even though there is a claim that she is a virgin, we base the law on the very fact that she was married, and we assume that she is not a virgin. As the Yerushalmi says: even if she technically remained a virgin, there is a measure of grace and charm that belongs to a woman who was never married. The very fact that she was married, even if marital relations never actually took place, causes her to lose that freshness that accompanies a virgin. Therefore, even if she is technically a virgin, that status has lapsed, and her kesubah is only a maneh.
"Ve'ein lahen ta'anas besulim" - if it turns out that she is not a virgin, the husband cannot claim that this was a mistaken transaction and that the marriage was entered into on a false basis, since the assumption after a marriage is that despite any presumption of virginity, she is not a virgin.
The second part of the Mishnah:
"Hagiyores vehashevuyah vehashifchah shenifdu veshenisgayru veshenishtachreru - yeseiros al benos shalosh shanim veyom echad - kesubasan maneh" - this refers to a convert, to a captive taken by gentiles, and to a maidservant: this one was redeemed from captivity, this one converted, and this one was freed from her servitude. However, all of these changes - the entry into the Jewish people, the redemption, or the emancipation - occurred when she was already older than three years and one day, and at that age any relations she had leave their mark on her and remove from her the status of a virgin. Therefore their kesubah is a maneh, since they are presumed not to be virgins.
"Ve'ein lahen ta'anas besulim" - if it turns out that she is not a virgin, the husband cannot claim a mistaken transaction. A woman who went through these situations is presumed to be someone who is not a virgin, and this is the basis upon which their marriage was built. Even if the husband thought she was a virgin, he has no ability to claim that the marriage was entered into on a false premise and that he expected to find a virgin, since from the outset the assumption is that she is not a virgin.
In summary: in this Mishnah we learned of two situations in which a woman's kesubah is a maneh and the husband has no claim regarding virginity: a widow, a divorcee, or one who received chalitzah following marriage, who, although presumed to be a virgin, lost that status through the very fact that she was married; and a convert, a captive, and a maidservant who converted, were redeemed, or were freed after the age of three years and one day, who are presumed not to be virgins.