In Chapter 16, Undine and Peter van Degen, who are carrying on an affair, discuss his plan to leave for Europe. Undine, with her characteristic melodrama, is very unhappy with Peter for leaving her when she thought they had a mutual affection. Undine's reaction to this uses a paradox that is instructive to her character:
She paused, and then, with a quick upward sweep of the lids: "After all, it's better it should be good-bye—since I'm paying for another mistake in being so unhappy at your going."
"Another mistake? Why do you call it that?"
"Because I've misunderstood you—or you me." She continued to smile at him wistfully. "And some things are best mended by a break."
Undine is indeed paying for multiple mistakes here. She doesn't want Peter to break off their affair, since she genuinely likes him and enjoys his attention. But she also feels like it was a mistake to believe in the first place that Peter was attracted to her. She feels like she has wasted her time, and she regrets cheating on Ralph with someone who was going to leave her in the end anyway.
After Undine realizes this, she gives a paradoxical statement: "Some things are best mended by a break." This is a logical contradiction. This paradox underlies much of Undine's character. She thinks, throughout the novel, that by breaking things (mostly, relationships with men) she can mend other things (her happiness). Throughout the novel, she leaves her relationships with men in much the same way that Peter leaves her here, trying to mend with a break. But only here, when she is the one who gets left, does she "wistfully" consider the consequences of her worldview, as represented in the paradox.