Another moment from the fifth book is how George and Harold defeat Wedgie Woman. By disguising several cans of hair remover as spray starch. The funniest part is that not only does Wedgie Woman lose her hair, but so does everyone else! Well, except for Captain Underpants, who was already bald to begin with.
Harold: Aaugh! My mom's gonna lay hard-boiled eggs when she sees me! George: Relax. Our hair will grow back! Harold: That's easy for you to say. Your hair was only half an inch long!
Shrunken down George and Harold, while stuck in tar, pick a real bad time to use Buffy Speak.
WE'RE GONNA GET STEAM ROLLER THINGY-ED TO DEATH!
P.E.T.T: People for the Ethnical Treatment of Toilets. In fact, all of the disclaimers before the Flip-O-Rama in each book.
From the intro comic to The Sensational Saga of Sir Stinks-a-Lot:
One day George and Harold had a really BIG problem. So they stole [which is crossed out] borrowed a time machine.
The copyright page for the first book has a hidden gem: a small box that says "Sturgeon General's Warning: Some material in this book might be considered offensive by people who don't wear underwear." Capped off by a drawing of a fish with a cap and medal next to it!
Mr. Krupp: Oh, no! My new name is "Lumpy Pottybiscuits!"
In the second book, George and Harold propose making a robot urinal called the Urinator to fight the Talking Toilets, only to decide that "they would never be able to get away with that in a children's book". Made even funnier by the fact that this conversation was taken verbatim from one between Dav and his girlfriend over his original idea for the book.
After Melvin and Mr. Krupp suffer a "Freaky Friday" Flip in the seventh book, George and Harold pull up a mirror to prove it to them. This exchange occurs:
Mr. Krupp (in Melvin's body): I'm... I'm a kid again! Melvin (in Mr. Krupp's body): And I'm old and fat and bald and ugly, and I have bad breath and creepy nose hairs and... Mr. Krupp:HEY!