The entire drunk scene. The fact that Vetinari's character is just ever so slightly off-key leads up to a reveal which truly deserves a CMOF.
Especially when someone completely and utterly pissed out of his mind tries to pat him on the back. Vetinari catches his hand in midair and reveals an encyclopedic knowledge of the man's football team's standing. After which the guy passes out.
Vetinari: I would just like a couple of people to take him back to his home. I would like them to put him to bed and see that no trouble comes to him. Perhaps they ought to stay with him until morning too, because he just might try to commit suicide when he wakes up.
Rincewind trying to get out of football practice. First he says he has a note from his mother, but Ridcully remembers that, to his continuing puzzlement, Rincewind once told him that his mother had run away before he was born. Not missing a beat, Rincewind still tries to weasel out of it:
Rincewind: Permission to go and find my mother?
The Bengo Macarona football chant, adjusted (at Macarona's insistence) to include all his academic appointments and titles. An Overly Long Gag as well, since after a couple repetitions the list has gone on for two pages. Worth slogging through if you like puns.
When they are discussing the offside rule, the football captains can't make sense of it. Funny, because in real life, even many hardcore football fans don't fully understand it.
Glenda's reaction to Mr. Nutt's botched attempt at flattery is to excuse herself, then march off to the library and demand "I need to see a dictionary of embarrassing words beginning with F!"
When dragons belch, and hippos flee, My thoughts, Ankh-Morpork, are of thee, Let others boast of martial dash, For we have boldly fought with cash, We own all your helmets, we own all your shoes, We own all your generals—touch us and you'll lose. Morporkia! Morporkia! Morporkia owns the day, We can rule you wholesale. Touch us and you'll pay.
Nyer ner ner nyer, nyer ner ner (And so on.) We can rule you wholesale, credit where it's due!
Mr. Nutt's final bit of training for the team has the team go without dinner, causing them to scramble together to find any food they can including a piece of cheese from a mouse trap and some of Macarona's Grandmother's Reheatable Pasta (with alligator testicle), and some of the Librarian's bananas:
Rincewind: The librarian will have some bananas.
Macarona: Are you sure?
Rincewind: I think the Librarian has a motto in these cases: "If you try to take my bananas from me, I will reclaim them from your cold dead hands."
Also Ridcully trying to ignore the fast and the tobacco ban, only to find Mrs. Whitlow the maid emptied his tobacco jar and cool cupboard (read: Discworld-equivlent of a mini-fridge), his emergency peppermints hidden in his jacket and finally his secret stash contained in a book, only to find another note along with his stash:
"Dear Archchancellor", I just didn't have the heart. Mrs. Whitlow"
Also his reactions to it, starting with a fairly calm "bugger", but after he finds his peppermints are missing:
Ridcully: Change and decay! I am surrounded by traitors!"
Pepe's thoughts on drinking water when Glenda offers him some water when he asks for a drink:
Pepe: I'll drink water when fish climb out of it to take a piss, but thank you all the same.
After shaming the dwarf merchant Glang, Trev and Nutt demonstrate their ball to him, complete with its perfect *gloing* sound as it bounces.
Glang: That was my... my grandfather's name.
During the book, Macarona is revealed to be gay during an awkward speech when Stibbons tries to explain. Later, during football practice Macarona shows to be adept at it. We get this gem.
Macarona seems to be very adept at handling the ball.
Ridcully: I'm not surprised.
When Lady Margalotta tells Mr. Nutt that he'll be teaching the other Orcs how to be civilized, he asks who she's sending to teach the humans. This little quip actually makes the normally stoic Vetinari burst out laughing.
This is likely a shout-out to a (possibly apocryphal) remark by Mahatma Gandhi upon leaving Britain after a speaking tour. A journalist asked him what he thought of western civilisation, and he replied, "I think it would be a good idea."
Alf Nobbs, who we are constantly reminded is of no relation to Corporal Nobby Nobbs.
Glenda saying "It's about your strip, sir. I mean, your uniform. Two U's, right here, like U.U.? You'll look like you've got bosoms!"
Vetinari replying to "She has a complaint. She is a maid" with "Tell her I can't help with that. Has she tried using a different perfume?" I.E. he took it as Drumknott saying that Glenda's complaint was being unmarried. (He meant that Glenda was a servant and had a complaint.)