Gittin, Chapter 8, Mishnah 7. This mishnah is essentially similar to the previous one, and deals with a case of yibum (levirate marriage): a co-wife who went and married a man from the marketplace rather than waiting for yibum, assuming that she was exempt from yibum, and in the end it turned out that she was mistaken - and on the basis of that mistake, penalties are imposed on her.
As we mentioned in the previous mishnah, this is stated according to the view of Rabbi Akiva, who holds that the prohibition of a yevamah marrying a man from the marketplace - the prohibition against the yevamah marrying anyone other than the yavam - produces a mamzer. And because the child is a mamzer, all the penalties are applied to this situation.
The halachic background:
When a man married to two wives dies without children, each of his wives is bound by yibum. However, once one of the wives has undergone yibum, the second not only need not undergo yibum, but is actually forbidden to do so, and she is permitted to marry another man. All of this holds only on condition that the mitzvah of yibum was in fact fulfilled through her co-wife.
The wording of the mishnah:
"Hakones es yevimto" - a man who married his yevamah, one of the two widows.
"Vehalchah tzaratah venisas le'acher" - her co-wife went and married another man, a man from the marketplace, assuming that the mitzvah of yibum had already been fulfilled through her co-wife.
"Venimtzeis tzaratah ailonis" - and in the end it turned out that the co-wife who had undergone yibum was an ailonis, barren, and was never bound by the mitzvah of yibum at all. It follows, then, that the mitzvah of yibum was never fulfilled, and therefore the second wife was not permitted to marry a man from the marketplace.
The law that emerges from here:
"Teitzei mizeh umizeh" - she must be divorced both from the man from the marketplace whom she married and from the yavam. "Vechol haderachim ha'eilu bah" - all the penalties listed in the two previous mishnayos apply in this case as well.
In summary: in this mishnah we learned about a co-wife who married a man from the marketplace assuming that the mitzvah of yibum had been fulfilled through her co-wife, and in the end it turned out that this co-wife was an ailonis and was never bound by yibum at all. Once the assumption on which she married was nullified, then according to Rabbi Akiva, who views the child as a mamzer, she must leave both this man and that one, and all the penalties listed in the previous mishnayos apply to her.