Particularly how they were contagious. Polly didn't have any idea what helicopters were, hadn't even heard the term ... but she could almost hear the rotors and nearly saw them coming into the "LZ"....
The entire sequence where the girls dressed as boys dress up as girls to sneak into the enemy's fortress. And nearly don't pass. In particular:
"Did you bring a weapon, Tonk — Magda?" "No, Polly." "No item of any sort with a certain weaponlike quality?" "No, Polly." "Anything, perhaps, with an edge?" "Oh, you mean this?" "Yes, Magda." "Well, a woman can carry a knife, can't she?" "It's a saber, Magda. You're trying to hide it, but it's a saber." "But I'm only using it like a knife, Polly." "It's three feet long, Magda." "Size isn't important, Polly." "Nobody believes that."
When the soldiers dress as washerwomen in order to sneak into the hold, the only one who convinces the guards that he's a female is Blouse, while the others are declared to be "obviously male". The hilarious part is that Blouse is the only actual male in the group.
And how Shufti proves her (and presumably everyone else's) gender, and Polly's admission later that "Things got a bit hairy for a moment..."
The description of how knowing too much about birds rather spoils one's enjoyment of birdsong: "It's hard to be an ornithologist and walk through a wood when all around you the world is shouting 'Bugger off, this is my bush! Arrg, the nest thief! Have sex with me, I can make my chest big and red!'
Nuggan's Abominations. And Vimes agreeing with him about the accordion players.
When the squad encounters a group of enemy soldiers, beats their asses, steals their clothes (and their boots), chains them up and drops the key into the privy - made doubly karmic and hilarious by the leader, Captain Horentz, turning out to be Prince Heinrich, ruler of the opposing country (and a total asshole). It's made even more brilliant when William de Worde turns up with Otto von Chriek and, to quote the article they wrote,