The Cunning Man crosses it in the readers' eyes (in case you didn't realize how bad he was) with the canary. More importantly, that's when he crosses it for Tiffany. It's the reason she accepts that he's beyond redemption, and no longer human enough for her to hesitate about vanquishing.
Nightmare Fuel: The Cunning Man. He is the personification of blind hate and lynch mobs.
Relationship Sue: You have to wonder a bit about Preston, especially the way he shares Tiffany's obsession with words.
They Wasted a Perfectly Good Plot: You would think the existence of a human girl with kelda abilities would be a big deal, but it has absolutely zero effect on the plot.
Kelda abilities are implied to be another form of witchcraft, closer to Nanny Ogg's methods than the Granny Weatherwax form Tiffany uses. Combined with Letitia's bookish magic (more like Magrat's), it serves to make the Chalk coven a mirror of the original Lancre coven.
It's also probable that Pratchett intended Amber's abilities as plot fodder for a later Tiffany Aching book.
Indeed, between Amber's kelda abilities, Letitia's self-taught skills, Annagramma's fireballs from Wintersmith, and the return of Eskarina, this book may be implying that magic has no clear boundaries between what's "wizard magic", what's "witch magic", what's "Feegle magic", etc: that such limits are imposed by magic-users' own assumptions, not inherent in magic itself.