Any time George and Harold rearrange the letters of a sign so that it says something ridiculous.
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".
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: 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!
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)
Mr. Krupp hiring new lunch ladies is probably a lot funnier for grown-ups.
Mr. Krupp: Do you have any experience? Alien: No. Mr. Krupp: Do you have any credentials? Alien: No. Mr. Krupp: Do you have any references? Alien: No. Mr. Krupp: You're hired!
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
When Melvin (as the Bionic Booger Boy) sneezes, he ends up splattering the chalkboard with mucus. Ms. Ribble advises him to cover his mouth... which ends up making things ten times worse as it turns him into a snot-based Action Bomb.
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: