Sotah, Chapter Three, Mishnah Three
The Mishnah has now arrived at the critical moment, right before she is about to drink the bitter waters. She is given one last opportunity to refuse to drink or to admit to her crime.
1) If before [the writing on] the scroll had been rubbed out, she said I refuse to drink, her scroll is stored away and her meal-offering is scattered over the ashes.
a) And her scroll is not valid to be used in giving another sotah to drink.
2) If [the writing on] the scroll has been rubbed out and she said I am defiled, the water is poured out and her meal-offering is scattered over the ashes.
a) If [the writing on] the scroll had been rubbed out and she said I refuse to drink, they open her throat and make her drink by force.
Section one: She may refuse to drink the water any time until the writing on the scroll has been rubbed out onto the water. At this point she need not admit that she had been defiled through adultery. However, she does lose her ketubah, as she would had she refused to drink at any other point in the process.
At this point, the scroll has already been written and her meal offering has already been prepared. The scroll cannot be used by another sotah, rather it is stored away. According to the Palestinian Talmud, it is hidden in the hinges of the doors to the Temples entrance. There it will become worn out quickly. The meal offering cannot be eaten or used for another purpose; rather it must be burned by spreading it out upon the ashes (but not upon the altar).
Section two: If the writing had already been rubbed out, she may no longer refuse to drink the water without admitting her guilt. If she does try to do so, she can be forced to drink the water. However, even at this very late stage she may admit her guilt and thereby avoid drinking the water. If she does so, the water is spilled out and as before, the meal offering is burned on the ashes. We can see that the rabbis wanted to do just about everything possible to allow her to avoid drinking the water.