Yevamot 9 - Why only fifteen cases?
Rabbi Yehudah the Prince takes the extra words "to her," and learns a lesson completely different from our previous one : "to her" refers " to the error (feminine) " that one commits when he worships idols, and the use of the same words connects the two areas of the law and teaches when and what sacrifice one must bring.
Levi asked Rabbi Yehudah the Prince, " Why did the central rule of Yevamot " teach only fifteen cases? It should teach sixteen!" Rabbi Yehudah replied, "It seems like this one has no brains in his head!" But what sixteenth woman could Levy have meant? The Talmud suggest that it may be a daughter of a man who violated a woman and whom his son married - but discards the idea, since this case is actually a disagreement, and the rule of fifteen was unanimous. It also suggests many others, much more involved relationships, but discards them all and explains why Rabbi Yehudah would not agree to any such scenario as a sixteenth case.
Also, the phrase "this one has no brains in his head" implies that the answer should be obvious, and that Levy should have deduced the answer on his own. Thus, the phrase in itself carries additional information, and the Talmud discussed what it might be.
Art: Fisherman and his Daughter by Charles Hawthorne