Vetinari arresting Mr. Gilt and the rest of the board members of the Grand Trunk at the end.
Moist Von Lipwig, whenever he decides to put all his chips in. Then, he tops it, when he puts in the chips he doesn't actually have. Then goes for double or nothing.
Moist's entire existence is pretty much one continuous CMOA, but the very best has to be at the end, during the entire monologue about how fear is hope turned inside out. "You know it can't go wrong, you're sure it can't go wrong... But it might." And then he looks at Reacher Gilt and thinks "I've got you."
The leadup to this sequence also deserves mention — the reader is fooled into thinking that Moist is going to go along with the Smoking Gnu's awesome plan to destroy the clacks network with their Steam Punk "computer virus", ...The Woodpecker (the ellipsis is key), as this is so lovingly described and would be thematic poetic justice, as it would not have worked if Gilt and his cronies hadn't destroyed the Grand Trunk's working culture — but Moist realises the chaos that would ensue and so quickly makes up a plan that brings down Gilt without damaging the clacks network. Even if he later morally agonises over using his con artist skills to imitate the voices of the dead.
Possibly the best thing about that is Moist's grasp of psychology. Even if Gilt is driving the Grand Trunk's working culture into the ground, the clacksmen out in the towers cling to their professional pride, and if a few of them do their job diligently, the Woodpecker will fail. Moist's alternative message is one the clacksmen are very happy to pass along.
The 'burning clacks towers' scene is also this trope for Pterry himself, as he cons the readers into thinking it actually happened, in a novel which stars a con-artist protagonist. Nice one, Sir Terry!
Moist realizing that he can bring his entire amassed fortune from years of crime into play, publicly, without incriminating himself in any way, by just telling the right lie. And then doing it.
Adora Belle's threatened use of very, very pointy heels. Do you feel lucky, jerk?
Vetinari has a hilarious one at the beginning of the book (Chapter one - In which our Hero experiences Hope, the Greatest Gift.) As the warder explains, Vetinari insists that all condemned prisoners should be offered the prospect of freedom (Only the prospect, not actual free freedom as such. That would be daft.) It provides occ-you-pay-shun-all ther-ra-py, healthy exercise, prevents moping and offers the greatest of all treasures, which is Hope.
And in the end, this gets that bastard Reacher Gilt killed. "Sadly, he did not believe in angels."
"And that's when it all went wrong for Mr Gryle, because Stanley had one of his Little Moments".
Vetinari trying his best to misunderstand business terms to make the Grand Trunk board members relax, because, after all, what did he know about business practices, he'd been classically educated. Then giving them just enough time to remember that his classical education had been at the Assassins' Guild.