Anvilicious: Dav will never let you forget that kids need imagination and fun in their lives and it plays as much of a role in their future as having an education.
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 TurboToliet 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.
Growing the Beard: The series started out as just simple harmless children's entertainment and little else. However, by the 4th and 5th books, Dav's commentary and messages become clear to more observant readers, with just enough subtlety to keep smaller readers entertained but still encouraged.
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.
In the first book, they ruin a football game For the Evulz, and when Mr. Krupp catches them doing it, he lists off some of their other major offenses during his Evil Gloating (putting bits of dissected frogs in jello at a parent-teacher banquet, making it snow in the cafeteria, rigging the intercoms to play loud music), which makes their subsequent punishment seem more like Laser-Guided Karma since they went out of their way to deserve it.
In the second book, they ruin a science fair out of spite because they were (rightly) banned from it due to a prank they pulled the previous year, making their punishment well-deserved.
In the third book, they sadistically torment the science teacher simply because they find his class boring. Then they pull a nasty prank on the lunch ladies (the only faculty members who aren't shown to be jerks) that covers the entire school in slime for no reason whatsoever, and as they are complaining to the principal, the lunch ladies list off the boys' other unprovoked offenses before resigning from their positions in disgust.
To be fair on the slime case that caused the lunch ladies to quit, George and Harold only intended to make it a small prank. The lunch ladies were the ones who decided to multiply the recipe by 100, causing the slime incident. Never mind the spot check they failed by not realizing what happens when you mix vinegar and baking soda, something taught in Fourth Grade the very same day.
The fourth book makes it a bit more complicated. While Professor Poopypants was going to crack at some point anyway, making an offensive comic book about him was just plain rude. In fairness, George and Harold do admit at the end of the book that they were wrong to make fun of him.
In the fifth book, they trick their homeroom teacher and principal into almost getting married. Sure, Ms. Ribble is a Sadist Teacher, but considering the magnitude of this action, her decision to simply have them flunk the 4th Grade comes off as an extremely lenient punishment (especially since their grades aren't all that good to begin with), yet they have the nerve to feel sorry for themselves.
To be fair on that last one, the whole situation could've been avoided if either Ms. Ribble or Mr. Krupp owned up to the fact that they didn't want to be married to each other some time before the actual wedding. George and Harold had nothing to do with the both of them failing to do that.
The Woobie: Kindergartner Harold. His dad's gone before the book starts, he's bullied on a daily basis, and it's all but stated he draws constantly because of everything that's happened to him. It's no wonder he's friends with George after the kid saves him.