Sotah, Chapter 4, Mishnah 4. This mishnah deals with several situations in which one might have thought that the woman cannot drink the bitter waters, or that the waters have no power to permit her to her husband. The mishnah teaches that even in these situations she drinks, and the waters permit her.
"Eishes kohen shosah u'muteres l'ba'alah":
The wife of a kohen drinks the waters and undergoes the entire process, and if she is innocent - meaning the waters do not kill her - she is permitted to her husband by Torah law, exactly like any other woman.
Why would one have thought that the wife of a kohen is different? Because in the case of the wife of an ordinary Israelite, if she was violated by the adulterer against her will, she remains permitted to her husband; whereas the wife of a kohen, even if she was raped, is forbidden to her husband. Since she was intimate with someone who is not permitted to her, this is considered a relationship of adultery that forbids her even when it happened against her will.
Accordingly, one might have reasoned as follows: it is true that the waters may not have killed her because she is free of guilt and was not intimate willingly, but perhaps she was intimate unwillingly - and then, being the wife of a kohen, she should be forbidden to her husband despite her innocence. The mishnah therefore teaches: we do not suspect that her innocence stems merely from the fact that she was raped; rather, we assume that she was not intimate at all, and therefore she is permitted to her husband.
"Eishes saris shosah":
The wife of a saris - who is not merely infertile but is missing the relevant organs - also drinks. Here is why one might have thought that she is not included in the passage: when the verse speaks of the adulterer, it says that someone other than her husband lay with her - from which one could infer that her husband must be a man who is capable of lying with her. One might therefore have thought that the status of a husband does not apply to their marriage, and that the passage of sotah does not apply to them. The mishnah teaches: if he warned her and she secluded herself with that man, she drinks the waters.
"Al yedei kol ha'arayos mekan'in":
Even if the potential adulterer about whom he warned her is an arvah to her - a man to whom she would be forbidden even if she were not married, because of family relationship and the like - the warning is valid, and if she secluded herself with him she drinks.
And here is the initial reasoning: since there is a halachah that a woman who committed adultery becomes forbidden both to her husband and to the adulterer, one might have thought that the entire passage of sotah applies only to a woman who, had she not committed adultery, would never have been forbidden to the adulterer - that is, if she were widowed or divorced she could marry him. And since she could never marry this particular adulterer, he being an arvah to her, one might have thought that the passage of sotah does not apply to her at all. The mishnah teaches that even though the man about whom he warned her is an arvah, the law of sotah does apply to her.
"Chutz min hakatan u'mimi she'eino ish":
This is the only exception, and it contains two cases:
The minor: if he warned her about a child. Regarding the definition of "minor," the Rishonim disagree:
One who is under the age of nine, whose physical act of intimacy is not considered an act of intimacy at all.
Anyone who is under the age of thirteen.
According to each of these views, these are the cases in which the law of sotah does not apply.
One who is not a man: for example, an animal. If a man warns his wife not to seclude herself with a particular animal, and he does not know whether she was intimate with it, the law of sotah does not apply.
In summary: in this mishnah we learned three cases in which one might have thought that the law of sotah does not apply, and the mishnah teaches that it does apply: the wife of a kohen - we do not suspect that her innocence is only because she was raped, and she is permitted to her husband; the wife of a saris - even though her husband is incapable of lying with her, the passage of sotah applies to them; and a warning may be given about any of the arayos, even though she could never marry the adulterer. The only exception is a warning about a minor (and the Rishonim disagree about his definition) and about one who is not a man, such as an animal.