Sotah, Chapter 6, Mishnah 4. This mishnah deals with a case in which witnesses disagree about whether the woman became tamei - that is, whether she was unfaithful during the time she was secluded.
The text of the mishnah:
"Ed omer nitmeis ve'ed omer lo nitmeis, ishah omeres nitmeis ve'ishah omeres lo nitmeis - haysah shosah" - and this is the explanation:
"Ed omer nitmeis" - one witness testifies that the woman committed adultery.
"Ve'ed omer lo nitmeis" - another witness contradicts him and testifies that she did not become tamei.
"Ishah omeres nitmeis ve'ishah omeres lo nitmeis" - the same law applies even when those who testify are women: one testifies that she became tamei and the second contradicts her.
"Haysah shosah" - in both of these situations the woman drinks the bitter waters.
Why is the first testimony not accepted?
Ordinarily, when we believe a single witness, once he has testified his testimony is treated as though it were that of two witnesses, and when a second witness comes and seeks to contradict him, we do not believe him. The mishnah here, however, is dealing with a case in which both witnesses came at the same time, and therefore the testimony of the one who said she became tamei was never established at all, since it never had a chance to take hold. But if the witness who says she became tamei had come first, she would not be given the waters to drink, because we would believe his testimony.
One witness against two witnesses:
"Echad omer nitmeis ushnayim omrim lo nitmeis - haysah shosah" - the two witnesses come and say to the first witness: we were there at that very moment, and what you said is not correct. Even so, there remains the possibility that she became tamei afterward, once one of us had left the place, and therefore she does drink.
"Shnayim omrim nitmeis ve'echad omer lo nitmeis - lo haysah shosah" - here we believe the two, and she is not given the waters to drink, for we have before us full testimony that she became tamei. And since she herself knows that she is guilty, there is no purpose in her drinking.
In summary: in this mishnah we learned that when the testimonies contradict each other on equal terms - witness against witness or woman against woman - the woman drinks, because they came at the same time and the testimony of tumah was never established. Even one witness against two does not exempt her from drinking, since the testimony of the two does not rule out the possibility that she became tamei after they left. But when two testify that she became tamei and one contradicts them, the testimony of the two is accepted, and she is not given the waters to drink.