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.
Also 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...
It may be a case of this reader being a bit slow rather than Fridge Logic, but 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.
At some point, the reader realizes that the graphic featured on the title page and that separates blocks of text throughout the book is the Summoning Dark rune.
And what happens when you shut the book? You leave that rune in the dark...
Well, as noted above, the Summoning Dark first has to choose a champion. Remember how Vimes got chosen? He injured his hand on a door with the symbol on it, and it seems like he was "infected" with the Summoning Dark then. So technically, you should be safe from it... hold on, is that a paper cut on your hand? That you got from reading ''Thud!''? Hope you have a badass mental watchman like Vimes...
NO! Don't! Remember the Summoning Dark that Angua painted on the drinks menu? Remember why Vimes can't just burn it? Because that sets the Summoning Dark free and releases it into the world
