Designated Hero: George and Harold have a tendency to slip into this with some of their pranks. In the first book, they cause enough disruption at a school football game to get the school to forfeit it, and subsequently ruin everyone else's day, and Captain Underpants is later created as a result of their attempt to avoid being punished for this. In the second book, they sabotage the school's Invention Convention out of spite, due to them not being allowed to attend following the prank they pulled last year. And in exchange for fixing all the damage done by the villains, they have Mr. Krupp cancel their punishment and let them run the school for the day, during which they sell a lot of the staff's furniture to pay for the party they throw for all of the students. Granted, they do try and stop the villains in every book, but the fact that many of them (the Talking Toilets, Poopypants, Wedgie Woman) appear as a result of George and Harold's actions doesn't help their case.
Granted, there were unordinary circumstances. Poopypants/Tinkletrousers was eventually going to crack and the fact it happened at Jerome Horwitz was just a matter of chance. With Wedgie Woman, they were unaware of the fact that the Hypno-Ring has reverse effects on women. Also, the villains of the 3rd book, the aliens, was completely unrelated to them at all.
These only apply to the first five books. As the books continue, we see more outside threats not by them or by returning villains (the latest book bringing back the Turbo Toliet 2000.)
Fridge Brilliance: Book 12 received a lot of praise for revealing that Future Harold is gay without making a big deal about it. Future George meanwhile has a wife that is only ever referred to as "Mrs. Beard". "Beard" of course is a term referring to a woman that has married a gay man.Have fun with that, fanfic writers!
In Wrath of the Wicked Wedgie Woman, it's revealed that the ring was recalled because for unknown reasons, it worked the opposite of its wielder's intentions on women. Unfortunate Implications aside, this might seem like jumping the gun to younger readers, but George and Harold used it on their already-Sadist Teacher and look what happened. Now, what about all the other kids who had one? Chances are, the first thing a lot of them would use it on is their parents, who they typically have a very low opinion of. All of a sudden, there'd be an outbreak of Abusive Mothers all across the country because the kids demand that their every whim be catered to. That wouldn't go unnoticed and would be a pretty damn good reason to recall them.
Hilarious in Hindsight/Heartwarming in Hindsight: In the library where the boys traveled to a different dimension, one of the books parodies the controversial children's book, Heather Has Two Mommies. Then in the 12th book, Harold marries a man with two children.
It Was His Sled: Thanks to the internet, there's no hiding the fact that Harold is gay in the 12th book.
Jerkass Woobie: Professor Poopypants. He just wanted people to stop laughing at his name, though George and Harold point out that he could have simply changed it rather than forcing everyone else to change their name into something ridiculous.
Nausea Fuel: Inevitable in Toilet Humour-themed series, but the Bionic Booger Boy gets special mention. His descriptive introduction is cut off when George yells at the narrator for making everyone sick.
The Bad Future created by Tippy Tinkletrousers. On top of this disaster-layer cake, we also saw gigantic zombie nerd-versions of George and Harold. Also from this section of the book, Tippy is crushed, blood and all. Doubles as a Tear Jerker.
The children affected by the Rid-O-Kid 2000.
The Scrappy: Melvin and some of the adults at Jerome Horwitz Elementary are this to a lot of fans.
Taken Up to Eleven in (appropriately enough) Book 11, where the boys convince all of the teachers and faculty that they're in a dream. This prompts the grownups to release their inhibitions by performing all sorts of shenanigans in their underwear. And yes, Dav Pilkey illustrates this repeatedly...
It's revealed that Harold's father walked out on the family when he was six, and Harold created the Captain Underpants comics as a coping mechanism. Mr. Krupp in his Captain Underpants identity is a kind of Bumbling Dad Parental Substitute for both of the boys, but particularly Harold...
When Tippy returns back to the present in the ending of the book, he's learned that thanks to his actions Mr. Krupp never became Captain Underpants and the previous villains succeeded in their goals. Just as he's about to return to set right what went wrong, he's note seemingly crushed by a giant, zombified Harold. The final pages of the book are just shots of smoldering wreckage.
After Yesterday George and Harold get brainwashed, George and Harold overhear their parents noting how differently they've been acting...and agree they like it.
Captain Underpants lost his powers and thanks to Sir Stinks-A-Lot wiping the hypnosis from his brain, he reverted back to Mr. Krupp permanently. Even worse, Yesterday George and Yesterday Harold decide to retire the character's comics in favor of making Dog Man comics, likely ensuring that except for a few people (George, Harold, Melvin) everyone will forget him.