Mamzer ממזר

The word is commonly misunderstood. A mamzer is not a child born out of wedlock: a child born to an unmarried mother is entirely fit. A mamzer is one born of a union for which the Torah prescribes excision: from a married woman with another man, or from close kin.

The weight. "A mamzer shall not enter the congregation of the Lord, even to the tenth generation"—that is, the bar to marriage within the people holds for the descendants. The one punished is not the one who acted but the one who was born, and the sages do not smooth this over. The midrash on Ecclesiastes, on the tears of the oppressed, says outright: these are the mamzerim, and they have no comforter, for they did not sin.

What was done. It was not abolished—and not justified. The law was built so that to declare someone a mamzer became all but impossible: a presumption of fitness for everyone born; a missing husband is not counted dead, yet the child of a woman who has come back is not hastily impugned; a doubtful case is decided in the child's favour; a court is not obliged to investigate and usually does not. There is a plain rule too: a learned mamzer takes precedence over an ignorant high priest.

And why this is in a reference book. Because here one sees how halakhah works at all. It does not repeal what is written and does not pretend that it is not hard. It narrows the application until it nearly vanishes—and leaves the verse standing.