Acting for Two: Upon repeat viewings, you'll notice that Cree Summer, Dee Bradley Baker, Tom Kenny, and Jennifer Hale voice a lot of characters.
Numbuh Four is voiced by Dee Bradley Baker as well as dozens of other extra roles, including villains (notably the Toilenator), operatives, and background characters.
Both Numbuh One and Numbuh Two are voiced by Ben Diskin.
Cree Summer voices both Abigail and her sister Cree.
The Delightful Children from Down the Lane's voice actors are three of the Sector V voice actors (Cree Summer, Ben Diskin, and Dee Bradley Baker).
Numbuhs 83 and 84 of Sector W are voiced by Janice Kawaye.
Adored by the Network: This show was a staple on Cartoon Network in the mid-2000s getting reruns very frequently. Even after it ended, it got reruns for years and it was one of the few shows to air both on Cartoon Planet and by itself. Boomerang also gave it good treatment by being one of the few shows to air consistently on the channel (Boomerang, in particular, is notorious for their inconsistency in airing reruns of several shows).
Alan Smithee: Because GWAR is certainly not a kid-friendly band, they are credited as "RAWG" in "Operation: F.O.O.D.F.I.T.E.".
Approval of God: There are a few posts on Mr. Warburton's blog complimenting various cosplayers or sometimes fanartists.
Contest Winner Cameo: Numbuh 24 from the episode "Operation: P.R.E.S.I.D.E.N.T." was modeled after Jhouwel, who won the contest "Operation: C.A.R.T.O.O.N.", the prize being a cameo appearance on the show.
Ben Diskin (voice of Numbuh 1 and Numbuh 2) has gone on record that "Operation: M.U.N.C.H.I.E.S." is his favorite episode of the show.
Both Ben Diskin and Cree Summer consider "Operation: S.U.P.P.O.R.T." to be one of their favorite episodes, as they found the jokes regarding Battle Ready Armor hilarious.
Surprisingly for a show with a cast comprised mostly of children, there are only some instances of this trope: Numbuh 84 voiced by Janice Kawaye, Numbuh 363 voiced by Jennifer Hale, the Numbuh 44 twins both voiced by Lauren Tom, Numbuh 88 voiced by Cree Summer, Willard and Baby Jackson both voiced by Tara Strong, and Numbuh 888 voiced by Kim Mai Guest.
In the Japanese dub, Numbuh 5 is voiced by Shūhei Sakaguchi, a man.
In the failed pilot "Kenny and the Chimp", Tom Kenny plays Kenny.
Dawson Casting: None of the actors voicing the kids are actual children, including Ben Diskin. When he voiced Numbuhs 1 and 2 on the pilot short "No P in the OOL," Diskin was already 18 years old.
Development Gag: Numbuh 5 wears reading glasses in "No P in the OOL". She doesn't do it in the actual show, but her DNK counterpart from "Operation: P.O.O.L." and her uncool self from "Operation: C.A.R.A.M.E.L." wear glasses.
Executive Meddling: A positive example: Originally, the Kids Next Door were to use high tech equipment to fight adult tyranny. The folks at Cartoon Network, however, asked this to be changed since Dexter's Laboratory had already done this. This resulted in 2x4 technology, which Tom Warburton has called one of the coolest things about the show. The show as a whole came into existence due to executives becoming interested in some of the side characters in "Kenny and the Chimp" and pushing Warburton into creating a second pilot starring these characters. These characters being what would become Sector V of the Kids Next Door.
All the characters on the show who hail from non-North American countries are voiced by Americans or Canadians. Ben Diskin as the British Numbuh One, Dee Bradley Baker as the Australian Numbuh Four, Jennifer Hale as the Scottish/Irish Numbuh 86, etc.
Japanese-American Numbuh 3 is voiced by Lauren Tom, who is of Chinese descent.
Ink-Suit Actor: Billy from "Operation: C.L.O.S.E.T.", one of the one-time-characters Tom Kenny voiced, looks like him as a child.
Keep Circulating the Tapes: It's been released on DVD a few times, but those were as part of a "Best Of" compilation, not in complete seasons or as a complete series set. The entire series is available on iTunes and the Playstation Network though, and the first 3 seasons have been put on Netflix.
The entire series, minus the Billy and Mandy crossover and "Operation I.N.T.E.R.V.I.E.W.S." (The Series Finale!), was added to HBO Max in January 2021.
Meanwhile, Boomerang's streaming service has both the crossover and I.N.T.E.R.V.I.E.W.S with the rest of the series, but not Z.E.R.O.
Make-A-Wish Contribution: A patient named Jacob wished to be in an episode. Jacob appeared as a scientist codenamed Numbah 275, albeit only a few times and with no spoken lines, in the second half of "Operation: F.E.R.A.L.". As a tribute, entering his name in the now-defunct KND online database made one receive the simple message of "awesome".
Method Acting: Whenever Tom Kenny recorded his lines as Knightbrace, he would put one of his fingers in his mouth in order to have the effect of wearing a retainer.
Numbuh Two's mother was voiced by Cree Summer in "Operation: T.O.M.M.Y.", but is instead voiced by Candi Milo for the rest of her appearances.
When Mr. Mogul and Big Brother appeared in "Operation: M.O.V.I.E.", they were respectively voiced by Dee Bradley Baker and Daran Norris rather than Rob Paulsen and John DiMaggio.
This Polish dub seems to be pretty bad in this. These are just the few examples:
Once Jacek Braciak (who voiced Numbuh 2 in the Polish dub) became a Celebrity Voice Actor somewhere in late mid-2000's, he was replaced with Łukasz Lewandowski starting from "Operation: W.H.I.T.E.H.O.U.S.E.".
While Tommy is normally voiced by Józef Mika, for Season 3, first half of Season 4 and Operation: Z.E.R.O. he is voiced by Janusz Zadura instead.
Numbuh 86 was first voices by Lucyna Malec in Season 2. In Season 3 she was replaced with Ilona Kuśmierska and from season 4 onwards the character is now voiced by Izabela Dąbrowska instead.
Numbuh 362 was initially voiced by Elżbieta Kopocińska in Season 2, before her role was taken over by Agnieszka Kunikowska in Season 3.
Delightful Children in particular have no consistent set of actors in Polish dub - every few episodes or so at least one actor in the group is replaced with another. For example, Leszek Zduń provided Polish voice for Lenny in "Operation: U.N.D.E.R.C.O.V.E.R.". Meanwhile, when the aforementioned character was temporarily recommisioned with the rest of Delightfuls in "Operation: Z.E.R.O." he was voiced by Krzysztof Szczerbiński, with Zduń voicing David instead.
Father, who is usually voiced by Jacek Rozenek, is voiced in Season 4 by Grzegorz Pawlak instead.
Chad was voiced by Paweł Iwanicki in the first appearances. In "Operation: U.N.D.E.R.C.O.V.E.R." and "Operation: E.N.D." he was voiced by Janusz Wituch instead. In Season 3 (and Operation: W.H.I.T.E.H.O.U.S.E.) he was replaced with Cezary Kwieciński. Beginning from Operation: E.L.E.C.T.I.O.N.S. he is voiced by Jacek Kopczyński (except for a cameo in Operation: P.A.R.T.Y. where the character was voiced by Krzysztof Szczerbiński instead).
Mr. Boss was first voiced by Jan Kulczycki in Polish dub. He was then replaced with Mariusz Leszczyński in Season 2, followed by Zbigniew Konopka in Operation: A.F.L.O.A.T. and Jarosław Boberek in Operation: U.T.O.P.I.A. Leszczyński voiced the character again in Season 4, while in the last few seasons and Operation: Z.E.R.O. Kulczycki returned for the role.
Stickybeard for the first two seasons was voiced by late Mariusz Leszczyński. In Season 3, he had two Polish voice actors - Paweł Sanakiewicz in Operation: A.F.L.O.A.T. and Zbigniew Konopka in Operation: T.R.I.C.K.Y.. For season 4, the character was voiced in Polish by Jan Kulczycki, while for the last two seasons Leszczyński returned for the role.
While Knightbrace for most of time is voiced by Wojciech Paszkowski in the dub, his sole line in "Operation: M.O.V.I.E." is provided by Marek Robaczewski. Similar things happened with Toiletnator (normally voiced by Robert Tondera) who was voiced by Cezary Kwieciński in Operation: Q.U.I.E.T.
Common Cold was voiced by Jarosław Domin in the first season. After the character was Demoted to Extra from that point, his grunts in Operation: A.W.A.R.D.S. were provided by Jarosław Boberek instead.
Heinrich Von Marzipan was voiced by Jacek Wolszczak in his first appearance, before being replaced with Tomasz Bednarek (who also regularly voices Numbuh 4) in "Operation: R.A.B.B.I.T.".
Real-Life Relative: The boy who gets abducted by the Abominable Snowsuit in "Operation: C.L.O.S.E.T." and his mother are voiced by Tom Kenny and his wife, Jill Talley.
The first video for the G:KND "teaser" was apparently leaked. For a time, typing in "Morbidcrab" (the Youtube account that leaked it) on the Rainbow Monkey site gave the message "why you gotta did that?". In the second video released after the countdown Numbuh Vine yells at Numbuhs Three and Four for posting the video early.
When writer Mo Willems left the show to become a kid's book author, he wrote "Operation: M.A.U.R.I.C.E.", an episode about a beloved operative leaving the Kids Next Door. "Maurice" is Willems' given name, just to drive the point home further.
According to this blog post, Mr. Warburton was surprised they managed to get away with the line "well, that figures" after Numbuh 86 turns into a female dog (i.e. a bitch).
Similarly, he said he was very surprised that Cartoon Network had virtually no notes regarding Nigel and Hoagie wearing Cree's bras in "Operation: S.U.P.P.O.R.T.".
When giving trivia about "Operation: I.N.T.E.R.V.I.E.W.S." on his blog, Warburton said he was really surprised to have got away with some of the Toilet Humor in the amusement park, with the "Drop-A-Log" roller coaster a notable stand-out.
Welcome to...: In "Welcome to Tumblr," the narrator says that Tumblr is a place that accepts men and women of all ages, but a person with a mustache and a pink bathrobe complains because that sounds like it's forcing people to conform to one of two derogatory gender roles. The narrator changes it to "Tumblr is a place for everyone," but the Delightful Children show up and complain that he only said everyone, which doesn't account for them since they're a hive mind consisting of five people.
Throw It In!: This is how they came up with Mr. Fizz — during the first season recording sessions, Jeff Bennett absent-mindedly began doing an impression of Ricardo Montalbán; Mr. Warburton knew he needed to put that in the show somewhere.
What Could Have Been: There were originally plans for this series to have a theatrical film following The Powerpuff Girls Movie. But due to that film underperforming, Cartoon Network backed away from feature films and have since stuck with made-for-TV movies for their shows, with this series getting Operation: Z.E.R.O. in 2006.
as well as gold and silver, making Captain Stickybeard a more realistic Pirate than he may seem at first glance.