It is the third Mishnah in the tenth Perek (on 92a in the Gemara). The husband and the son went overseas and the wife was told that her husband died first and then her son died, so she remarried. Later, she was told that really the son died before the husband. The Mishnah states that the children she bears from the second marriage are Mamzerim because in reality she was a Yevamah l'Shuk when she remarried.

(However, this is not the Halachah, as the Rambam (Hilchos Yibum v'Chalitzah 3:19) writes. In the above case, the children from the second marriage are not Mamzerim, because the Halachah follows the view of the Rabanan (and not Rebbi Akiva) who maintain that no Mamzer results when only a Lav is transgressed through the marriasge.)

Kol Tuv,

Dovid Bloom