- Any time George and Harold rearrange the letters of a sign so that it says something ridiculous.
- Book 1:
- A florist's shop that says "Pick Your Own Roses" becomes "Pick Our Noses".
- The school sign saying "See Our Big Football Game Today" becomes "Boy Our Feet Smell Bad".
- Book 2: A furniture store sign that says "Come In And See Our Pretty Armchairs" becomes "Come and See Our Hairy Armpits".
- Book 3: The school menu advertising "New Tasty Cheese And Lentil Pot-Pies" becomes "Nasty Toilet Pee Pee Sandwiches".
- Book 4: The school sign that says "Don't Underestimate Our Good Teachers" becomes "Our Teachers Don't Use Deodorant".
- Book 5:
- A school bulletin board that says "People—Please Wear Your Socks On The Gym Floor" becomes "Please Go Pee-Pee On Your Socks For Warmth".
- Another board that says "Have A Blissfully Grand Retirement, Ms. Ribble" becomes "Ms. Ribble Really Needs A Breath Mint".
- Book 6: A bulletin board saying "Please Wash Your Hands After Using The Toilet" becomes "Please Wash Your Hands In The Toilet". And Melvin falls for it!
- Book 1:
Book 1: The Adventures of Captain Underpants
- 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!
Book 2: Attack of the Talking Toilets
- One chapter ends with George and Harold saying they have to catch Mr. Krupp before it's too late and he starts turning into Captain Underpants—the next chapter begins with said Captain running down the hall crying jubilantly about underwear. The looks on George and Harold's faces are PRICELESS. Appropriately enough, the chapter in question was named "It's Too Late".
- 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.
- George and Harold propose making a robot urinal called the Urinator to fight the Turbo Toilet 2000, 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.
Book 3: Invasion of the Incredibly Naughty Cafeteria Ladies from Outer Space (and the Subsequent Assault of the Equally Evil Lunchroom Zombie Nerds)
- One of the warnings before a Flip-O-Rama suggests that if you find the chapter too disturbing, you should run to your nearest shoe store and order a cheeseburger (it doesn't make sense, but it will be funny). Later:
Captain Underpants: I was at the shoe store ordering a cheeseburger.
Book 4: Perilous Plot of Professor Poopypants
- Almost all of the names from the Name Change-O-Chart 2000 are hilarious.
Mr. Krupp: Oh, no! My new name is "Lumpy Pottybiscuits!"
- 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!
Book 5: Wrath of the Wicked Wedgie Woman
- "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. The accompanying illustration is a close-up on their faces to show that their noses look identical.
- The rabbi who comes to marry Mr. Krupp and Ms. Ribble in Wedgie Woman telling George and Harold that he doesn't want any tricks today. Harold's response? "Silly rabbi! Tricks are for kids!"
- 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!
Book 6: Big, Bad Battle of the Bionic Booger Boy, Part 1: The Night of the Nasty Nostril Nuggets
- The introduction of the Bionic Booger Boy is Nausea Fuel incarnate, with increasingly graphic descriptions of just how Squicky the whole ordeal really is — to the point that George yells at the narrator to knock it off.
Ms. Ribble: Thank you, George.
Book 7: Big, Bad Battle of the Bionic Booger Boy, Part 2: The Revenge of the Ridiculous Robo-Boogers
- After Melvin and Mr. Krupp suffer a "Freaky Friday" Flip, 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!
Book 8: Preposterous Plight of the Purple Potty People
Book 9: Terrifying Return of Tippy Tinkletrousers
Book 10: Revolting Revenge of the Radioactive Robo-Boxers
Book 11: Tyrannical Retaliation of the Turbo Toilet 2000
- When Mr. Krupp sees the faculty running around in their underwear, creating havoc in the halls, all he can say (again) is "B-b-b-bubba-bobba-hob-hobba-hobba-wah-wah."
Book 12: Sensational Saga of Sir Stinks-A-Lot
- From the intro comic:
One day George and Harold had a really BIG problem. So they stole [which is crossed out] borrowed a time machine.
- This exchange in Super Diaper Baby after Deputy Dangerous is turned into Deputy Doo-Doo:
Deputy Doo-Doo: Man, this hill is steep! I'm so tired!Danger Dog: Are you pooped?Deputy Doo-Doo: Why, yes, I'm... hey! Shut up!Danger Dog: Aw, don't be a party pooper!Deputy Doo-Doo: I said SHUT UP!Danger Dog: When we get home, will you read me "Winnie the Pooh?"Deputy Doo-Doo: I'LL KEEEEEL YOU!
- Earlier, when Deputy Dangerous finds out he's turned into poop, he looks in the mirror. He yells, "I'm a piece of poo!" and his reflection yells, "He's a piece of poo!"