In "The Vexing Villainy of the Vile Vimpire", some of the sad things Captain Underpants says while under the influence of the Vimpire's bite (i.e. "I started going bald in my teens!", "Everyone forgot my birthday!", "Ive never felt truly loved!") sound a bit out of character for him, considering his personality and (what he thinks is) his backstory. But then this becomes brilliant (and a massive Tear Jerker) when you realize that these aren't Captain's memories... theyre Krupp's.
The Family-Unfriendly Aesop given at the end of The Soggy Salvation of the Swirling Sweatnami (that 'some people are just born bad') actually gains another layer to it when you realize who delivers it: Erica. Given the fact that she (somehow) remembers the alternate timeline, she probably realized George and Harold were behind it and gave them this aesop as a way to dissuade them from trying to alter the past again.
There is a reason why Mr. Ree and Ms. Yewh hit it off as a potential couple. In Ree's second appearance, he wonders out loud if Robe-I-Con has any kimonos. Kimonos are a traditional style robe made and worn in Japan. From this, we can assume that Ree has an appreciation for Japanese culture, similar to Ms. Yewh who is known for her love of French culture. The bond that connects them is their love for different world cultures.
In the Season 1 Finale, its revealed that Superintendent Vil Endenemys is not only Melvin's future self, but also a cyborg. Which brings up the question, how did Future!Melvin get like that? Was he in a horrible accident, and had to be rebuilt in order to save his life? Or... did he do it to himself on purpose? Considering what Melvin was initially intending to do to himself before accidentally turning himself into the Bionic Booger Boy in Book 6... I wouldnt put the second option past him.
As of Season 2, Episode 7, it's actually the first option.
In episode 5, after catching them trying to T.P. the entire school, Mr. Krupp punishes the boys and tells them that after they clean up all the toilet paper, he wants them to mow the schools football field... with their teeth. Why would he come up with a punishment so unrealistic? Then you remember that back in the day, farm owners usually had their livestock eat the grass instead of using a lawnmower. Mr. Krupp grew up on a cattle farm. And then you remember what his mother is like. Bernice most likely forced him do that as a punishment at least once when he was young.