In "Operation: C.A.T.S.", Numbuh 5 claimed that she was a dog person. Then came Operation: D.O.G.H.O.U.S.E.... where she ended the honor roll student's curse of turning into homework-munching were-dogs, but not before becoming one herself.
At first, it was ok to not give sympathy to the Delightful Children From Down The Lane, but after the movie where we realized that they used to be Sector Z, a missing KND sector and were "delightfulized", it would mean that their "negative" counterparts, The Little Traitor Dudes For Children's Defense were once evil operatives of the Destructively Nefarious Kids or DNK, a negative counterpart of the Kids Next Door where they are devoted to tormenting children everywhere rather than saving them.
In Operation C.H.A.D., Mega Mom and Destructo Dad want to get rid of all members of the KND so that their son Numbuh 274 can become "Numbuh 1". While it would seem like an absurd and hilariously petty motive back then, it becomes a lot harder to watch come Operation T.R.E.A.T.Y. where it's revealed that Chad has developed a bitter Inferiority Superiority Complex with Numbuh 1, one that destroyed the friendship the two had, even in spite of Chad's betrayal of the KND being a ruse.
"Operation: E.N.D." has Chad turning out to be a traitor to the Kids Next Door entirely because it was his 13th birthday, and he opted to decommission every single operative on the mailing list. He had offered to also try to help Numbuh 1 earlier in the episode for clearing his team's names, but it's ambiguous if Chad actually meant it or was just going to betray him once they got to the Moon Base, but he went pretty far to Kick the Dog and seem as evil and petty as he could possibly be. Then we find out in "Operation: T.R.E.A.T.Y." that Chad was actually a false traitor, pretended to go evil to infiltrate the teens and their adult superiors — and was secretly supporting Sector V from behind the scenes, setting up their convenient victories and escapes. It's entirely possible that Chad was going to help the team and get them off of his decommission list before the big reveal, but Numbuh 1 went rogue, forcing Chad to improvise and end up getting caught by Numbuh 1 ahead of time for his troubles.
The KND's Splinter Cell was an extremist group who wants to get rid of adults bad AND good which the KND opposes. In Operation T.R.E.A.T.Y., the group are revealed at least and it turned out to be fakes made by Numbuh 74.5239 and Numbuh Infinity who are aligned with the Galactic Kids Next Door who are above even Soopreme Leader Numbuh 362 that fight against adulthood in space. Just as we knew about the Splinter Cell comes the teaser for the next possible sequel series... The G:KND has Nigel make the decision on activating something that would likely endanger the planet Earth in order to get rid of adulthood there once and for all which Chad begs him to not go through with it because of their friends and family still on the planet. To confirm Chad's fears, Numbuh Vine/Lizzie alerts Wally and Kuki to evacuate EVERYONE on Planet Earth, kid and adult alike! In a way, the Splinter Cell DOES exist and it is named the Galactic Kids Next Door who will take down adulthood even on the planetary scale!
There have been reports of 'troubled teen camps' that are practically concentration camps for kids instead of Jews. If there is any reason for the KND to exist in real life, it isthis.
Numbah 5's dad, who appeared in a few episodes, is a thinly-veiled Expy of Bill Cosby, right down to his jolly, somewhat absentminded rambling Verbal Tic. As with many works referencing Bill Cosby, such appearances have a much darker tone following the real Bill Cosby's sexual assault allegations in 2014.
"Operation: G.R.O.W.N.-U.P." has Father and a grown-up Numbuh One facing off, and, after Father comments on what a "disappointment" the DCFDTL are, Numbuh One responds with "that must run in the family". And then, in the same scene, Numbuh One says that "his Grandma can throw better than [Father]." Which, considering Father is Nigel's uncle, and considering that it turns out Grandfather actually does find Father to be a disappointment... yeah.
In the Kenny and the Chimp pilot, Kenny got... Swine Flu, a major epidemic in summer 2009.
In 2003, "Operation: P.O.P." aired, where the "Soda Control Act" forbids anyone under the age of 13 from drinking soda, forcing kids to "brew" soda illegally and sell and drink it in speakeasies a la the days of Prohibition. In 2013, a law was almost passed in New York City that would have forbade anyone from purchasing/selling drinks with a certain sugar content in any size bigger than 16 ounces. One can only wonder what the Kids Next Door were up to during the months of battling over the law's legality.
"Operation: T.R.I.C.Y.C.L.E." featured a character called Numbuh 10-Speed, a parody of Lance Armstrong. Not so funny anymore after the reveal that Armstrong used steroids to win his Tour de France titles. Lampshaded in the Galactic Kids Next Door ARG site, where entering Numbuh 10-Speed's name returns the response "undergoing testing".
"Operation: O.O.M.P.P.A.H.": In venting his frustration about being dragged along on this trip, Nigel says he wants to be with his friends and chastises Monty for doing all this because he read an article in the paper (essentially accusing him of needing someone else to tell him how to be a father). Monty sheepishly responds that he just wants to be friends with his son. This is played as a sad moment, but much later, it would get even worse, as we'd learn more about Monty's upbringing. He didn't have the kind of father who set a good example or wanted to be his friend.
this.