These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Creepy Cute: The titular lice is "Operation: L.I.C.E."—they eat hair and sport some rather impressive fangs, but the bugs are rather adorable. Especially the baby lice seen on Numbah Two near the end.
Crowning Music of Awesome: The rock opera episode, complete with a score by GWAR, who re-performed the song themselves after being unable to find stock copies.
Designated Villain: Done deliberately with Professor XXXL, who in all his appearances is just trying to create the perfect snowcone, but the KND always ruin his plans thinking he is doing something evil.
Numbuh ThreeÎFour were this in the first season thanks to fan reaction to "Operation: O.F.F.I.C.E.". The escalating Ship Tease in the subsequent seasons upgraded them into Official Couple status. Even the last episode revealed that they'd be husband and wife.
Numbuh 65.3ÎLizzie: They're both glasses-wearing geeks. In "Operation: L.E.A.D.E.R.", Lizzie pretends to flirt with him and says she likes his pocket protector, making 65.3 blush. In "Operation: G.I.R.L.F.R.I.E.N.D.", 65.3 tells Nigel to say "hi" to Lizzie for him.
"Operation: G.R.O.W.N.-U.P." has Father and a grown-up Numbuh One facing off, and, after Father comments on how useless 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... yeah.
The first comment doubles as Hilarious in Hindsight, considering that Grandfather actually does find Father to be useless.
In the Kenny and the Chimp pilot, Kenny got... Swine Flu.
"Operation F.U.T.U.R.E." is a Terminator parody, where girly girls take over the world from an all girls school and the last rowdy boys are fighting a losing battle against them. Many academic studies show that in Real Life the girls are increasingly taking over, due to being generally better students and that today's rowdy boys are going to be tomorrow's losers.
Tiger Moms — the battle against adult tyranny is as important as ever!
Growing the Beard: The Season 1 Finale established continuity for the series, as well as the introduction of Father and the fact that the KND is not just the five featured on the show, but rather many different sectors.
Harsher in Hindsight: In Kenny and the Chimp, one of the joke diseases that Kenny get is Swine Flu. And then in 2009, it came back (swine flu was around in the mid-1970s).
Chad following the reveal that his betrayal of the KND was a ruse. On one hand, he's still an incredibly egotistical and bitter person, to the point that he tries to kill Nigel in Operation T.R.E.A.T.Y. However, it's easy to feel some degree of sympathy for him given [[spoiler: the fact that he willingly tarnished his own reputation within the KND, and gave up all the friends he made it.
Memetic Badass: Just watch "Operation: K.N.O.T." and try to say Moosk doesn't have the makings of this.
Memetic Mutation: One fansite dedicated to re-captioning screenshots had characters discuss a clothing-optional resort. "The resort" has been a KND meme ever since.
One episode where "chicken pox" takes the form of literal squirming chicken heads sticking out of one's skin.
A villain named the Common Cold attacks by siphoning mucus out of his nose. Especially lovely when he starts to ready a really big attack. Probably why he was Demoted to Extra after his first 2 appearances.
In "Operation: P.I.N.K.E.Y.E.", being hit with a water balloon filled with pinkeye mucus would cause crusty red stuff to grow all over you. The episode's villain turns out to be using this crusty stuff as the crumbs in crumb cake. It gets even worse at the end of the episode: "So if the pinkeye was the crust, then what was the filling?" (achoo!)
Even worse? In two later episodes, Numbuh Two still eats the stuff and he comments about it as well!
Is no one going to mention "Operation: V.I.R.U.S.", where there is a close-up of Cree's gigantic and disgusting pimple?
A few of The Movie's Senior Citizombies have saggingGag Boobs.This included the citizombified versions of Numbuh Three and Five.
At the end of "Operation: C.A.K.E.D.-T.H.R.E.E., after the KND defeat the Delightful Children's Giant Birthday Present Robot with the help of an army of Chicks, Numbuh 1 is about to take the cake from the Delightful Children when the Chicks defecate all over it. Numbuh 1 then convinces the Delightful Children to eat the cake, which they do. It also counts as an in-universe example, as several children are grossed out from watching the Delightful Children eat the soiled cake on their television sets.
Seasonal Rot: ironically around the time the series began to establish its mythos and continuity, which was about season 2 and onward for some people when Flanderization took hold of Numbahs 3 and 4 despite all the character development everyone got, especially those two despite their Flanderization.
Ships That Pass In The Night: Numbuh 86 and Numbuh 60 is a rather popular ship, despite the fact that they never really had any onscreen interaction. (The closest was Operation: T.R.A.I.N.I.N.G., when Arctic Base got a distress signal from Moonbase. And even that turned out to be just Father disguised as Numbuh 86.)
They Wasted a Perfectly Good Plot: The KND "Splinter Cell" arc foreshadowed an internal civil war in the KND between the extremist Splinter Cell that believes all adults must be destroyed vs. the more moderate faction that believes only the evil adults should be stopped. It is also revealed that the Splinter Cell had been kidnapping KND scientists for unknown reasons. However, near the end of the series, it turns out that the Splinter Cell is just an front for the KND scientists, who fake their kidnappings so they have more free time to watch their favorite science fiction shows as well as manage "the Galactic Kids Next Door", the workings of which were left very vague in the end.
There's also the matter of the 1/362 Ship Tease. Despite Operation: I.T. showing how close they were and Operation: G.I.R.L.F.R.I.E.N.D. all-but-confirming that Rachel has feeling for Nigel beyond friendship, it was never resolved. The trope comes in with after Lizzie dumped Nigel, which looked like it was foreshadowing that he would end up with Rachel. You can imagine how many fanfics have centered around them getting together.
In the main team alone; 3+4 didn't so much have subtext as it was just plain text from the start, 2+5 was a slightly more debatable case, and 1+5 had some good chemistry. But let's not get into the web that the fans (and creators) have spun.
Villain Decay: Father started out as... well, pretty much Satan. Scary, always cloaked in darkness, and biblical in his wrath—and of course, his voice is Maurice LaMarche at his scenery-chewing best. Contrast this with the middle-aged dork he became later on.
Mega Mom and Destructo Dad were considered an extreme threat to the KND as a whole in their first appearance. It was also implied they took out the entire Kids Next Door (Not sure about Moonbase Operatives) with the exception of their son, Numbuh 274. Besides of the fact that often attend all of the Villain gatherings, their connection to Chad is never mentioned again, even after he becomes a Double Agent. Also, they are never seen using their powers in future appearances.
Woolseyism: All over the place in the Latin American dub.