When the smith is trying to figure out a power source for his combination harvester, and the pot boils over, creating a large cloud of steam. Then he bitches about how he always gets interrupted when he's trying to think.
Many, many books later, Raising Steam reveals that he did make the connection eventually. He didn't achieve much with the insight, but his son does.
Death getting the stereotypical flowers, chocolates and diamond ring as gifts for a lady, including him buying one of everything from a totally oblivious florist, interrogating the candy-seller because he knows nothing about chocolate (What for is this box padded? Is it to be sat on? Can it be that it is cat flavoured?) and invading a temple to steal the biggest diamond in the world because a jeweller told him it was the friendliest diamond he could think of (Death had been told, unwisely, that diamonds are a girl's best friend). The break-in involves the high priest, the other priest who was not high, and Indiana Jones jokes.
Made even funnier when the two priests, upon realizing that the intruder is making it past all of the traps, are afraid that it'll turn out to be "Mrs. Cake!"
Doubles as Fridge Brilliance, because Mrs. Cake's precognition means she probably could bypass all those traps without injury.
Pointing out that Miss Flitworth died when he came back to give her one last dance.
Bill Door: You know when you said that seeing me gave you quite a start? Mrs. Flitworth: Yes? Bill Door: I gave you quite a stop.
Bill Door: You know when you saw me and said I gave you a stroke? Mrs. Flitworth: Yes? Bill Door: Quite.
Reading the approach to the lair of the strange creatures and realizing that it's a shopping mall.
The footnote about shopping cart wranglers:
It is generally thought, on those worlds where the mall lifeform has seeded, that people take the wire baskets away and leave them in strange and isolated places, so that squads of young men have to be employed to gather them together and wheel them back. This is exactly the opposite of the truth. In reality the men are hunters, stalking their rattling prey across the landscape, trapping them, breaking their spirit, taming them and herding them to a life of slavery. Possibly.
The Auditors, after Azrael's answer.
Some of the roof fell in.
The wizards becoming action heroes: Yo! Hut!
Ridcully: And incidentally, if you say "yo" one more time, Dean, I will personally have you thrown out of the University, pursued to the rim of the world by the finest demons that thaumaturgy can conjure up, torn into extremely small pieces, minced, turned into a mixture reminiscent of steak tartare, and turned out into a dog bowl.
Dean: Y— Yes. Yes? Oh, go on, Archchancellor.
Chief Alchemist: I tell you, it's hell in my workshop! There's stuff whizzing everywhere! Just before I came out, a huge and very expensive piece of glassware broke into splinters! General Secretary and Chief Butt of the Guild of Fools and Joculators: Marry, 'twas a sharp retort.
Even funnier, the Fool has to be led away by kind people when the Chief Alchemist points out that it was actually an alembic.
Every time Ms. Cake starts to answer someone's questions before they ask them because of her 'premature precognition'. Perfect "WHAT" moment.
The wizards trying to bury Windle Poons because he was supposed to die, but death is missing.
"Can you keep it down? There's people down here trying to be dead!"