Creator Backlash: When the movie made Mr. Krupp less of a flat-out evil person along with giving him a Character Arc, Pilkey admitted that he wished he had done something like that in the books.
The European French translation of "The Perilous Plot of Professor Poopypants" was originally slated for a 2002 release, but that version never saw the light of day. A different translation, commissioned by another publisher, was finally released in 2014.
The 9th book was stuck in this for six years until finally being released in 2012, as Dav Pilkey took some time to care for his dying father.
At the end of Captain Underpants and the Preposterous Plight of the Purple Potty People, two additional books were teased, neither of which have been heard from since:
The Captain Underpants Cartoon-O-Rama: Book #1: Heroes, Villains, and Super Creeps, a how-to-draw book.
FrankenFart vs. the Bionic Barf Bunnies from Diarrhea Land, a book supposedly written by Evil Dav Pilkey from the Mirror Universe depicted in Book 8. (Although it is very possible that the inclusion of the book was just a joke.)
Dub Name Change: Most of the character names are changed in the Spanish (USA and international), French (Bayard), French (Scholastic Canada), German (Ueberreuter), Italian (though George, Harold and Melvin's names went back to English in reprints), Portuguese (Brazil, mostly the first names), Catalan, Danish, Swedish (Bonnier Carlsen; except Mr. Krupp and the students), Finnish, Icelandic, Polish (except Krupp and the children), Croatian, Slovenian, Russian (most of the teachers), and Hebrew (some characters).
Dr. Diaper's name was changed to Dr. Nappy in the UK for obvious reasons.
The series has had two European French translations: one by Editions du Petit Musc (which covered the first 3 books) and another by Bayard (covering the first six).
There are two German translations: one by Ueberreuter (which covered the first five books) and a newer one by Panini.
There are also two Norwegian translations. In one translation, George's name is "Thomas" while the other calls him "Georg".
There are two Swedish translations: one by Egmont, and another by Bonnier Carlsen (which covers all 12 books).
There are also two Italian translation: the first covered only the first 8 books, while the second (that is more of a slightly altered version of the old one, just removing the changed names for George, Harold, Melvin and a few other character) covers the entire series.
Line to God: Dav Pilkey has an Instagram, and has been known to not only answer fan questions, but likes and comments on several pieces of fanart based on his works.
Recycled Script: In book 6, Melvin tries to fuse himself with a robot, but sneezes during his Combine-O-Tron combination process, and gets combined with his boogers. This is similar to a plot line in Super Diaper Baby in which Deputy Dangerous also tried to fuse his body with Billy's super powers, but Billy soiled his diaper at the last second, and Deputy Dangerous got combined with Billy's poop.
Fans had to wait three years between the publications of books 7 (2003) and 8 (2006). After that, Dav Pilkey spent some years caring for his terminally-ill father. His next book, The Adventures of Ook and Gluk, Kung-Fu Cavemen from the Future (another George and Harold graphic novel) was finally released in August 2010, the first of a four-title publishing deal.
As noted above, book 9; The Terrifying Return of Tippy Tinkletrousers, didn't come out until 2012, six years after book 8.
George and Harold initially hypnotize Krupp because they were fed up with him blackmailing them with a VHS tape that has dirt on them.
In the Flip-O-Rama segment of book 4, Harold references CGI as being a new and novel form of animation (which, during the book's release of 2000, it was).
Torch the Franchise and Run: It was all but stated that this was meant to be the case at the end of "The Terrifying Return of Tippy Tinkletrousers", where Tippy accidentally messes up the timeline and causes a paradox, creating a universe where Captain Underpants never existed, ending with him supposedly getting squished to death by George and Harold, who are now giant zombie nerds, and the narration implies that this book was supposed to end the series. Fortunately, Pilkey retconned this and gave the series three more books and a proper conclusion.
According to an old update on Dav's website, The Terrifying Return of Tippy Tinkletrousers was originally going to be Part 2 of The Preposterous Plight of the Purple Potty People, part 1 entitled, The Clash of the Colossal Kung-Fu Monkeys from Beyond Infinity. However, due to its Development Hell status the plot dramatically changed, so that Purple Potty People is treated as its own story, while Tippy Tinkletrousers focused more on George and Harold's kindergarten days. This meant the actual plot involving Tippy having revenge on George, Harold and Captain Underpants ended up being told in The Revolting Revenge of the Radioactive Robo-Boxers.
Rip Van Tinkle and Petey from Super Diaper Baby 2 were originally called Prime Minister Pee-Pee (later Pee-Pee Longstockings) and Ralph early in development.
Four additional spin-off books were announced, but never saw the light of day at some points:
The Captain Underpants Cartoon-O-Rama Book: Heroes, Villains, and Supercreeps, seemingly an activity drawing book. Given how the cover has a “#1”, the book was likely meant to kick off a series of other how-to-draw books.
Fraken Fart vs. The Bionic Barf Bunnies from Diarrhea Land, first appeared as an in-universe book in the eight Captain Underpants book. 14 years after being announced, it went on to appear again as an in-universe comic in the first Cat Kid Comic Club book, also written by Pilkey.
The Adventures of Ook and Gluk Jr: Kung-Fu Cavekids in Outer Space, a sequel to The Adventures of Ook and Gluk: Kung-Fu Cavemen from the Future, cancelled entirely after the first book was pulled from publication in 2021.
Captain Underpants Presents: F.A.R.T.S. The Major Motion Picture, an “epic novel and film combo”.
Write What You Know: George and Harold have many similarities to Dav Pilkey when he was a child. Like George and Harold, Pilkey grew up in Ohio, was diagnosed with ADHD, and would constantly get into trouble with his teachers for drawing comic books, which themselves were based on things that were happening to him at school. Pilkey even originally created the character of Captain Underpants when he himself was in elementary school.
Write Who You Hate: According to Dav Pilkey himself, Melvin Sneedly, the stuck-up nerd, is based on an arrogant tattletale child that Pilkey Dav knew in sixth grade, who has been given the alias "Michael Sneedman" to conceal his identity.
, and has been known to not only answer fan questions, but likes and comments on several pieces of fanart based on his works.