The local police officer stops by to chat with Mrs Wilberforce, but since the gang have told her to drive him away without making him suspicious, she doesn't invite him in as she normally would. Instead she blurts out all the excuses the gang had suggested— starting out with "You can't come in here without a search warrant!" and ending with "Now would you please buzz off!" (thanks, Harry).
Mrs. Wilberforce briefly forgets the suitcase (with the loot in it) that the gang is using her to pick up.
driving past the police station, they see their case of money sitting in the doorway
One-Round's losing it when he thinks someone's killed Mrs. Wilberforce ("I said nobody was to do Mrs. Lopsided!"), culminating in a brawl that comes to a shocked stop when Mrs. Wilberforce appears at the foot of the stairs (she was asleep).
Mrs. Wilberforce: What are you doing? And who is "Mrs. Lopsided," may I ask?
Professor Marcus' death by railway semaphore. In the stage version this is part of a Chekhov's Gag, characters keep treading on his scarf, which he then struggles to pull free. At the close, Marcus flees down a train tunnel, only to find his scarf caught yet again and... well you can guess what happens next.
The 2004 film:
The General's plan to murder Ms. Munson backfires hilariously when a cuckoo clock startles him into swallowing his lit cigarette, then he tries to drink the water containing her false teeth, which makes him bolt out in terror, tripping over the cat and breaking his neck as he falls down the staircase. All set to an old-timey gospel tune.
Professor Dorr's Oh Crap reaction when Lump tries to think for himself.
Ms. Munson's barrage of slaps to Gawain's face upon hearing a sample of his "hippity-hop" language.
Anything Gawain says and/or does:
This bit, after Garth tells him about his IBS and won't stop talking about it:
Gawain: Can't stop shittin', can't stop talkin' about shittin'...