Yevamot, Chapter Fifteen, Mishnah Nine
This mishnah continues to deal with a woman who returns from overseas and states that both her husband and son have died. The difference between this case and that found in the previous mishnah is that here her son was born abroad.
1) [If a woman states], A son was given to me [while I was] in a country beyond the sea and she also states, my son died and afterwards my husband died, she is believed.
2) [If she states], my husband died and afterwards my son died, she is not believed, but we are concerned that her words [might be true] and she must, therefore, perform halitzah but may not contract yibbum.
Section one: In this mishnah she leaves to travel abroad under the assumption that she is liable for yibbum should her husband die, because he has no children. If she comes back and states that she had a son, she has changed that status quo. However, if she says that the son died before the husband, then she reverts to the status quo of being liable for yibbum. Since she is ultimately maintaining the status quo, she is believed.
Section two: However, if she states that her husband died before her son died, she is changing the status quo because she is exempting herself from yibbum. Therefore, she is not believed, and she is not allowed to remarry without first having halitzah. She cannot have yibbum because she claims that her husband died before her son and therefore she is not liable for yibbum.
In summary both this mishnah and yesterdays mishnah teach that a woman is believed to retain the status quo but not to modify it. However, even though she cannot change status quo, she must have halitzah before she can remarry.