Captain Underpants: I was in the shoe store ordering a cheeseburger.
Another chapter ends with George and Harold saying they have to catch Mr. Krupp before he turns into Captain Underpants, but then the next chapter begins with said Captain running down the hall crying jubilantly about underwear. The look on George and Harold's faces are PRICELESS. Appropriately enough, the chapter in question was named "It's Too Late".
"B-b-b-bubba-bobba-hob-hobba-hobba-wah-wah." The only thing Mr. Krupp is able to say when he learns he's getting married.
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!