Sotah, Chapter 1, Mishnah 3. As we mentioned earlier, a woman for whom there was kinui (warning) and setirah (seclusion) and who became a sotah, may not eat Terumah as long as she has not drunk the sotah waters, if she is the wife of a kohen. But once she has drunk and been found pure and remained alive, she is permitted to her husband and permitted to eat Terumah. Our Mishnah discusses the women who remain forbidden to their husbands and forbidden to eat Terumah.
"V'eilu asuros mile'echol biTerumah":
The Mishnah lists women who are permanently forbidden to eat Terumah, and who are also forbidden to their husbands - and if their husband is a kohen, they are forbidden Terumah as well, and this applies even if the husband is not a kohen:
"Ha'omeres teme'ah ani lach" - a woman for whom there was kinui and setirah, who does not wish to drink, but says: "Indeed, I committed adultery." Since she does not actually drink the sotah waters, she is forbidden to her husband. And even though we learned that a woman who tells her husband that she is forbidden to him because she had a forbidden relationship must bring proof, since we do not believe her, that applies only when she says so out of nowhere, all of a sudden. Here, by contrast, there is "raglayim ladavar" (grounds for the claim) - the fact that there was kinui and setirah - and therefore she is believed and is forbidden to her husband.
"V'shebau lah eidim shehi teme'ah" - if witnesses came and said that she had relations. Certainly if the witnesses came before she drank the waters she is forbidden, and even without kinui and setirah: once two valid witnesses came and said that she committed adultery, she is forbidden to her husband, and in a case where there was kinui and setirah, even one witness is enough. Our Mishnah teaches the novel point that even if she drank the waters and appeared to have been found innocent, and afterwards two witnesses came and said that she committed adultery - they are believed and she is forbidden to her husband. Once there are two witnesses, there is testimony.
Why did the waters not affect her?
There are two possible reasons for this:
As we will learn later in the tractate, there are opinions that hold that merit protects her. If she has the merit of Torah, from her support of her husband's Torah or her children's Torah and the like, that is enough for her not to die from the sotah waters.
Some understand from the verse that the waters take effect only when there is no witness. That is, if witnesses exist somewhere in the world, the waters do not take effect on their own; and since these witnesses existed, that is the reason the waters did not affect her.
In any case, once witnesses came and said that she committed adultery, she is permanently forbidden to her husband and forbidden to eat Terumah.
The Mishnah continues and lists:
"V'ha'omeres eini shosah" - a woman who says: "I am pure, but I refuse to drink." We do not force her to drink, and at the very least, before the Name of Hashem has been erased into the water we do not force her. But she is forbidden to her husband, she is divorced without her kesubah, and she may not eat Terumah.
"V'sheba'alah eino rotzeh l'hashkosah" - if the husband refuses, for whatever reason, to have her drink, she is forbidden to him. In this case she receives her kesubah, since it was the husband's choice not to have her drink, while she for her part was willing to drink.
"V'sheba'alah ba aleha baderech" - if the husband had relations with her on the way to beis din, to have her case judged. Such a woman is forbidden to him from the time of the kinui and setirah, and once he transgressed the prohibition and had relations with her unlawfully, the waters no longer take effect. The Torah says that the woman bears her sin only when the husband himself is free of sin; and since he transgressed the prohibition, the waters do not take effect, and therefore she remains forbidden.
"What is done with her?":
Since we have already noted the concern that the husband who brings her up to beis din might have relations with her, the Mishnah explains how we proceed: the husband first goes to the local beis din, and in order for them to go up to Jerusalem, two men are assigned to accompany him, and the husband is not allowed to travel with her alone, lest he have relations with her on the way.
Rabbi Yehudah disagrees and says that there is no need to assign him two men to supervise, because the husband is trusted to be careful and not have relations with her on the way. His reasoning is a kal vachomer from niddah: a woman who is a niddah is forbidden to her husband under penalty of kareis, and nevertheless her husband is permitted to be secluded with her; all the more so with a sotah, where there is only a negative prohibition and no kareis, that he should be trusted.
The Sages, however, say the opposite: the very fact that it is not such a severe sin is reason to be more concerned, and therefore guards must be sent along with him.