Sotah, Chapter 3, Mishnah 3. Our Mishnah deals with three situations in which the woman refuses to drink the sotah waters or admits that she is defiled, and it distinguishes between them according to the stage at which the words were spoken: before the scroll was erased or after it.
Before the scroll was erased and she said, "I will not drink":
The Mishnah opens: "Ad shelo nimchakah hamegillah, amrah: eini shosah" - that is, the scroll on which the portion of the sotah was written, containing the Name of Hashem, has not yet been erased into the water, and at this stage the woman announces that she does not intend to drink. In saying this she does not admit that she committed adultery; she merely refuses - and she is not forced to drink. She remains forbidden to her husband, and he is required to divorce her without a kesubah. But since the procedure itself will not take place, the Mishnah establishes two rulings:
"Megillasah nignezes" - the scroll is put away in genizah, in a safe place, and is no longer used.
"Uminchasah misfazeres al hadeshen" - the meal offering becomes invalid, since its entire purpose was for the procedure of drinking the bitter waters, and this woman is not going to drink them. "Hadeshen" means the ashes, referring to the place in the Temple courtyard where invalidated items are burned.
The Mishnah continues: "Ve'ein megillasah kesheirah lehashkos bah sotah acheres" - the scroll is not valid to be used for the drinking of another sotah. The source of this ruling is the verse "And the kohen shall carry out for her this entire law," from which the Sages expounded: "For her - this teaches that it must be done for her sake." Just as a get must be written for the sake of those particular divorce proceedings, so too the sotah scroll must be written for the sake of this particular woman, and therefore it is not valid for another.
The scroll was erased and she said, "I am defiled":
A second scenario: the scroll has already been erased, and at this stage she said: "Temei'ah ani" - the woman admits to adultery. At this point there is no longer any doubt, for the whole purpose of the drinking is to clarify the doubt as to whether or not she committed adultery, and she herself has clarified it. Therefore "hamayim nishpachin" - the water is poured out, "uminchasah misfazeres al hadeshen" - and her meal offering, which has become invalid, is burned in that same place in the Temple courtyard where invalidated items are burned.
The scroll was erased and she said, "I will not drink":
A third scenario: the writing on the scroll has already been erased, and the woman does not admit that she is defiled but says, "I will not drink." Here the Mishnah rules that we have learned from the verse that she is to be treated differently - me'arim osah - a stratagem is used with her, and she is made to drink the sotah waters against her will. Various explanations have been offered for the term "me'arim," among them that she is confused and disoriented, but the result is one: she is compelled to drink. Once the Name of Hashem has been erased into the water, her refusal is no longer of any avail.
In summary: The Mishnah distinguished between three situations. She said "I will not drink" before the scroll was erased - her scroll is placed in genizah, her meal offering is scattered on the ashes, and her scroll is not valid for another sotah, as derived from the exposition of "for her - for her sake." She said "I am defiled" after the erasure - the water is poured out and her meal offering is scattered on the ashes, since the doubt has been clarified by her own mouth. She said "I will not drink" after the erasure - a stratagem is used with her and she is made to drink against her will.