The Mishna on our daf introduces us to the halakhic argument: “Ha-peh she-asar hu ha-peh she-hitir – the voice that forbade is the voice that permitted.” In other words, when we are only aware of a potentially problematic situation because of someone’s admission, we trust that person to explain why the situation is, in fact, not a problem at all. Thus, if a woman walks into court and says “I was married, but have received a divorce,” we will accept her story and allow her to marry with no need for her to prove that she is now single. If, however, we knew that she was married based on other evidence, we cannot accept her word that she is divorced without some proof to that effect.
The Gemara quotes a baraita that goes one step further. The baraita teaches that a woman who says “I am married” can come to court afterwards and say “I am single” and will be believed if she gives an amatla – a convincing rationale for why she originally said what she did. In response to a query from Shmuel, Rav ruled that a married woman will also be believed if she says that she is permitted to her husband, even though the previous night she said that she was forbidden (i.e. that she was a nidda), as long as she gives a convincing explanation of her statement the previous night. The Gemara records that Shmuel accepted the ruling, although he did not apply it when it came up in a personal case.
Tosafot bring the Talmud Yerushalmi, which explains that Shmuel had turned to Rav with this question because one night his wife had told him that she was forbidden and the next night that she was permitted, explaining that she was simply too tired the night before to engage in relations and had excused herself by claiming that relations were forbidden. The She’iltot presents the story in a different way. According to that version, one of the hints that a wife would give to her husband to indicate that she was a nidda was that she would decline to drink from the cup of wine that her husband offered her. When this happened in Shmuel’s home, his wife later explained that she had declined the wine for another reason – she did not want to embarrass Shmuel’s sister, who was a guest at the table and had not received a cup of wine. It was in that case where Rav ruled that her explanation could be accepted.