Yevamot, Chapter Ten, Mishnah Three
This mishnah discusses other scenarios where a woman was told that her husband died and it turns out that the information she received was not 100 per cent correct.
1) If a woman whose husband and son went to a country beyond the sea was told, Your husband died and your son died afterwards, and she married again, and later she was told, the [deaths] happened in the opposite order she must leave, and any child born before or after is a mamzer.
2) If she was told Your son died and your husband died afterwards, and she had yibbum, and afterwards she was told, the [deaths] happened in the opposite order she must leave, and any child born before or after is a mamzer.
3) If she was told, Your husband died, and she married, and afterwards she was told, He was alive but is now dead, she must leave, and any child born before [the death of her first husband] is a mamzer, but one born after it is not a mamzer.
4) If she was told, Your husband died, and she was betrothed, and afterwards her husband appeared, she is permitted to return to him.
a) Although the second one gave her a get he has not thereby disqualified her from marrying a priest.
b) This Rabb Elazar ben Mathia expounded: A woman divorced from her husband (Leviticus 21:7)not from a man who is not her husband.
Section one: When this woman hears that her husband died while her son was still alive, she thinks that she is not liable for yibbum or halitzah, since her husband died while his son was alive. Therefore she remarried someone else, without halitzah or yibbum. When she finds out that she should have had yibbum, it turns out that she was a shomeret yavam who remarried without halitzah or yibbum. Our mishnahs ruling goes according to Rabbi Akiva who rules that any children from such a marriage are mamzerim. This is true whether the child was born while her husband was alive or even if the child was born after her husband died, for she should have had halitzah or yibbum. She must also leave this marriage.
Section two: In this case she thought that she was liable for yibbum, so she went and had yibbum with her brother-in-law. Afterwards she finds out that she was exempt from yibbum, since her husband died while her son was still alive. Since the yibbum was forbidden, any child that she has with the yavam is a mamzer, whether or not he was born while her husband was still alive.
Section three: In this case, at the point when she married the second husband, she was in fact married to the first husband. Therefore she must leave the second marriage. The child born before the first husband died is a mamzer, for the union was adulterous. However, the child born after the first husband died is not a mamzer. In other words, even though she must leave the second husband in any case, not every child that she has with him is a mamzer.
Section four: If she is only betrothed to the second husband, and then she finds out that she is still married to the first husband, she may return to her first husband. In the other cases in our mishnah and in the previous one, she was forbidden to do so because she had, albeit unwittingly, committed adultery by having relations with the second husband. However, in this case, she has not had relations with the second husband, therefore she may return to the first. The mishnah further emphasizes that the second marriage is totally invalid by stating that even if the second husband gives her a get, she may still potentially someday marry a priest.
This last halakhah is supported by a midrash by Rabbi Elazar ben Matya. Leviticus 21:7 states that a priest may not marry a woman divorced from her husband. The verse could have merely stated divorced. The extra words from her husband teach that if she is divorced from someone who is not her husband, a priest may still marry her.