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.
Then, when it's about to happen, Ms. Ribble suddenly opts out because of how silly his nose looks. Their noses look identical.
Returns in book 11 when Mr. Krupp sees the faculty running around in their underwear, creating havoc in the halls.
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.