Angua points out to Carrot, after returning from her trek through the mud, that the dwarf who painted the Summoning Dark rune on the door in his own blood was probably still alive when she and Vimes went down into the Long Dark mine with Ardent. Vimes gashed his hand on a nail/rivet on one of the water-tight doors while he was down there. Ardent caught up with him about that moment, quite flustered by Vimes' going down a tunnel without him. Ardent explained it as "this is a mine, there are dangers!" Once you consider the fact that somewhere down there, one of the miners was dying because of Ardent and the grags, it adds a little more color to Ardent's sudden concern.
Angua also mentions at the time that there wouldn't be a nail in a metal door. And leaving a rivet/weld sharp enough to cut yourself on is exceptionally poor engineering practice and totally uncharacteristic of dwarfs. The cut wasn't from anything physical - it was Vimes being "infected" by the Summoning Dark, in much the same way that it left a scar after it left.
Arguably the fundamentalist dwarves' attempts to prevent knowledge of the kings' message from spreading, when one realizes they were erasing recorded words.
One for longtime readers of the series: in the famous "THAT! IS!! NOT!!! MY!!!! COW!!!!!" line... well, look at it. One more exclamation point for each word. Remember how both Reaper Man and Maskerade discussed how five exclamation marks is a sure sign of madness? Well, it's a much less comedic use here since it's in the middle of Vimes' climactic struggle with the Summoning Dark, but still...
It's mentioned in the scene where Carrot, Angua, and Sally are investigating the crime scene that every deep dwarf has a draht, a unique identifier tattooed on their right wrist. The Summoning Dark leaves an "exit wound" scar on Vimes' right wrist - so from now on, any dwarf automatically looking for a draht would see the Summoning Dark.
After Vimes 'cuts' his hand on the 'rivet' in the door (see above), Igor goes on about 'tiny invisible biting creatures' (lisp omitted). Now, imagine that in the form of the Summoning Dark. A tiny invisible biting creature, eating its way into his mind...
There's a subtle moment during Vimes' talk with Mr. A. E. Pessimal: Vimes explains why he wants to take on the auditor Pessimal as an officer, summing up with 'I want someone who can hold a pencil without breaking it'. This may sound like a frustrated/affectionate jibe at the expense of Vimes' men, who are generally not at home to Mr. Paperwork. But when you notice that just a page previous to the exchange Vimes himself has literally broken his pencil by pressing too hard with it, you realise that it is also self-deprecation.
When Grag Bashfullsson starts wondering if the Summoning Dark is following the other grags. Later on, we realize that it is. Right inside Samuel Vimes.
When Vimes and co are bowling along to Koom Valley in the magic coaches, the narrative notes that the landscape ahead looks bluish, while the landscape behind has a "relatively red tint". They're slicing time.
They were most likely just moving at relativistic speed and are experiencing blueshift and redshift, which also occur while time-slicing. The speed of light on the Disc is quite low. Pratchett even uses relatively to describe the redness behind them.
The Dwarfs fanatically respect the written word and their lore. And yet, at some point since Koom Valley, their priest class systematically defaced written versions of their origin myth so that the part about Trolls was completely reversed in their culture's collective memory. That was the depth of their unthinking hatred: to defile everything they claimed to hold sacred in order to retroactively justify that hate. They probably thought they were heroes for doing it, too.
Troll teeth are made of diamond. Mr Shine is made of diamond. Mr Shine is made of teeth...
This Troper applies this logic thusly: Human Fingernails are made of Keratin, Human hair is made of Keratin, Humans grow fingernails from their heads...