This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Lords and Ladies
The King of the Elves could fix Lancre's problem. Nanny threatens him with The Magic Goes Away... obviously not an option a witch would lightly choose, but she'll do it if that's the only alternative.
"One day." Nanny nodded. "Yes. I'll drink to that. One day. Who knows? One day. Everyone needs 'One Day'. But it ain't today. D'you see? So you come on out and balance things up. Otherwise, this is what I'll do. I'll get 'em to dig into the Long Man with iron shovels, y'see, and they'll say, why, it's just an old earthworks, and pensioned-wizards and priests with nothin' better to do will pick over the heaps an' write dull books on burial traditions and suchlike, and that'll be another iron nail in your coffin. And I'd be a little bit sorry about that, 'cos you know I've always had a soft spot for you. But I've got kiddies, y'see, and they don't hide under the stairs because they're frit of the thunder, and they don't put milk out for the elves, and they don't hurry home because of the night, and before we go back to them dark old ways I'll see you nailed!"
The bit where we find out that Esme Weatherwax remained a virgin for the rest of her life after Mustrum Ridcully left Lancre.
Although it is quite strongly implied that it wasn't specifically because of Ridcully himself (actually, Granny pretty much states it outright, when she tells him not to jump to any soppy conclusions) but that the reason she rejected him and the reason she remained a virgin are one and the same - she had her mind set on other things. Whether Granny is being truthful here, and whether this is actually sad, are probably a matter of YMMV.
It's probably a little of both. She's happy with what she is and what she has, but what might-have-been isn't without beauty either.
Ridcully: Do you think that... somewhere... it all went right?
Granny: Yes. Here! (then, relenting a bit) But there, too.