When the Piecemaker is first introduced, inside a very small room.
Nobby: Those things are a bugger for metal fatigue, especially in the safety catch. Detritus: What are a safety catch? Everything went very quiet.
When Detritus and Cuddy go to see the alchemists and Detritus calls them 'loony bastards' to their faces in a very matter-of-fact way because that's what Colon called them earlier.
Nobby's absolute glee at seeing the Klatchian Fire Engine, as well as his insistence on using it.
Not to mention Nobby arming himself to the teeth with weapons and then proceeding to drop half of them.
And the footnote about the Fire Engine being banned in eight countries and three religions, while five more religions embrace it as a holy weapon to be used on infidels, heretics, gnostics, and people who fidget during sermons.
The Reveal of Big Fido, the fierce Hilter-Esque leader of the Dogs Guild, as a small fluffy poodle who knows next to nothing about how wolves really are.
The lengthy discussion about just what it is they say about dwarfs. When the answer turns out to be that they're well-endowed and just as you think the joke is that Cuddy misinterprets it as meaning "They have loads of gold," it's turned completely inside-out by the real punchline:
Nobby: You dwarfs really love gold, don't you?
Cuddy: Of course we don't. Don't be silly.
Nobby: Well -
Cuddy:We just tell it that to get it into bed.
A similar Literal-Minded joke with dwarfs: "Have you any previous convictions?" "Well, I used to very firmly believe that a penny saved was a penny earned."
Vimes' pun-based metaphors in the narrative when he muses on how Sybil and Nobby get along so well: it must be the attraction of opposites...
"The Ramkins were more highly bred than a hilltop bakery, whereas Corporal Nobbs had been disqualified from the human race for shoving."
The report Cuddy writes up for Vimes following his visit to the Alchemists guild with Detritus. Cuddy, being a dwarf, writes his police report in the manner of an epic poem. The opening line is especially hilarious.
Bright was the morning, and high our hearts, as we proceeded to the Alchemist's guild where events eventuated as I shall now sing.
Detritus has the smallest semblance of authority for about 10 seconds before he starts acting as a Drill Sergeant Nasty. It's hilarious, especially the "Slice Mountain" part.
After Carrot realizes Angua's secret a moment too late, he asks Colon if he had known. Colon admits that Vimes had hinted about it before. "He sort of said, 'Fred, she's a damn werewolf.'"
The "Truncheon" scene.
Colon:Hright! This, men, is your truncheon, also nomenclatured your night stick or baton of office. Hand you will look after hit! You will eat with hit, you will sleep with hit, you — Cuddy: 'Scuse me. Colon: Who said that? Cuddy: Down here. It's me, Lance-Constable Cuddy. Colon: Yes, pilgrim? Cuddy: How do we eat with it, sergeant? Colon: What? Cuddy: Well, do we use it as a knife or a fork or cut in half for chopsticks or what? Colon: What are you talking about? Angua: Excuse me, sergeant? Colon: What is it, Lance-Constable Angua? Angua: How exactly do we sleep with it, sir? Colon: Well, I... I meant... Corporal Nobbs, stop that sniggering right now!
The armory scene in general is one extended CMOF to anyone who's ever spent any time in any branch of any armed service, but particularly the bit with Carrot telling the Quartermaster that "If you feel unable to assist us, you have only to say the word and we'll leave," while the Quartermaster, who has a very large crossbow bolt pressing into his neck, is unable to think of any word except "FIRE!"