When Vetinari is telling Lipwig about the Royal Mint he asks him to look out of the window of the coach they are driving in:
Vetinari: What does gold know of true worth? Look out of the window and tell me what you see. Moist: Um, a small scruffy dog watching a man take a piss in an alley. Sorry, but you chose the wrong time.
Vetinari (nearly) getting hit by a clown's pie: "I do believe it is pineapple."
The impromptu auction of Pucci Lavish's one Dollar Bill.
Any scene involving Mr. Fusspot, from the mandibular acrobatics involved in him eating toffee to his use of a sex toy as a chew toy. ("That was not a rubber bone.")
Moist's intro to Pucci. He gets in the first cart he sees - which happens to be a "Honey Trap" organized by Pucci (she is in a state of dishabille and is trying to get a picture of Moist with her to use as blackmail). Moist promptly jumps out the other side of the carriage... while holding Mr. Fusspot in his arms.
Moist's explanation of why there are now mongooses in the letterboxes. They were introduced to keep down the snakes, which they put there to get rid of the toads, which they encouraged because of the snails, which got there themselves to eat the stamp glue. Vetinari then notes brightly that they were saved the trouble of introducing the snails. (This is doubly funny if you know how such similarly brilliant solutions worked out in real life. Which is to say, very poorly.)
How Vetinari reacts when Moist explains that he's not fit as a banker because he's robbed banks.
Vetinari: Capital! Just reverse your thinking. The money should be on the inside!
Doubly funny at the end when you learn that the bankers robbed the bank.
When Moist receives Mr. Fusspot, the sweetly-written letter from Topsy finishes with a warning to Moist that if the dog dies of unnatural causes, 'your arse will belong to the Guild of Assassins.' Moments later an arrow crashes through his window. Written on the shaft is a slogan that reads;
The Assassins' Guild - where style counts!
Equally funny is how casually Gladys and Moist treat it, especially when Gladys catches a second one between her fingers, inadvertently setting it on fire. "Someone Has Sent You An Arrow, Mr. Lipwig." "Oh, just blow it out and put it in my in-tray."
A rather... interesting moment from Cosmo: Pucci tells him that his obsession with Vetinari is a dangerous one. "I'm simply trying to get inside the man." "A very dangerous obsession." "You know what I mean!"
Pucci's departure from an epic failure to discredit Moist's new-fangled paper money: "The girl could flounce better than a fat turkey on a trampoline."
Harry King invests everything in the bank once Moist's trial starts. Moist's only thought is "It was a terrible thing to have Harry King believe in you."