"Rav Elazar ben Yaakov said: I heard that the beit din may hit and punish not according to the [laws of the] Torah, not to violate the words of the Torah, but to make a fence around the Torah" (Yevamot 90b).
"A woman whose husband went overseas, and one came and told her, 'Your husband has died' and she marries [another]..." (Yevamot 87b). Whereas Jewish law requires two witnesses in all matters of criminal and family law, when it comes to freeing a woman who is "chained" to her missing husband, this law is relaxed, allowing the woman to remarry based on the testimony of only one witness.
One of the key distinctions between the land of Israel and the Diaspora is the ability to observe the many mitzvot between man and the land. The seventh chapter of Yevamot focuses on the intricacies of the laws of terumah, which may be eaten only by a kohen or a member of his household. The chapter begins by analyzing the case of a kohen marrying someone prohibited to him, such as a divorcee.
"Shimon ben Azzai said: I found a scroll of genealogy, and it was written, 'So-and-so is a mamzer, [having been born] from [a forbidden union with] a married woman; and it was written, 'the teaching of Rabbi Eliezer ben Jacob is small in quantity, and clean'. And in it was also written, 'Menashe slew Isaiah'" (Yevamot 49b).
Life is not always fair. So much that befalls us--both positive and negative--is beyond our control. And there is little that is less fair than being born a mamzer, an illegitimate child. Through no fault of one's own, one is stigmatized for life, unable to marry most Jews. Such status is the result not of an unfortunate accident, but of a deliberate sin of the most heinous kind: adultery or incest by the mamzer's parents.
On Yom Kippur afternoon, the Torah reading focuses on the Jewish sexual ethic. The Torah has a relatively long list of relatives with whom intimacy would be considered incest. If one were to "marry" one of these people--say, one's aunt--such a union would be of no standing. There is a second group of prohibited marriages, issur kedusha, where one may not marry someone; but such a forbidden marriage, while a violation of the sanctity of marriage, would be recognized as valid.