Also, Frank Welker voices Professor XXX-L, who in the "Kenny and the Chimp" short, dealt with all kinds of crazy chemicals and diseases; furthermore, he's always got a different set of animal body parts, making him a sort of mutant. All of which seems a bit similar to Welker's character in SWAT Kats, Dr. Viper, who got mutated into a half-animal creature after dealing with an unstable chemical, and spoke with a similar voice (albeit with hissing as opposed to XXX-L's lisp, and it's a bit more sinister).
Contest Winner Cameo: In "Operation: P.R.E.S.I.D.E.N.T.", complete with no spoken lines, and an overly detailed face.
Cross-Dressing Voices: Averted; even though most of the boys are about 10 to 12, the voice actors are all adults who play their respective genders (Numbuhs 1, 2, and 4 are played by men and Numbuhs 3 and 5 are played by women).
Fan Nickname: Ashley, the sole unnamed girl of the Delightful Children.
Keep Circulating the Tapes: It's been released on DVD a few times, a couple of random "Best of" DVD's, but never in box-sets or a complete season. The entire series is available on iTunes and the Playstation Network though, and the first 3 seasons have been put on Netflix.
Reality Subtext: The first video for the G:KND "teaser" was apparently leaked; typing in "Morbidcrab" (the Youtube account that leaked it) on the Rainbow Monkey site gives 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.
Numbuh Three was originally going to speak only in Japanese (with subtitles), while Numbuh Five was originally going to be The Voiceless. The reason both those ideas were scrapped was because it would have resulted in both girls having no lines and possible protests over alleged sexism (and possible racism, since both girls aren't white [Abby is African-American and Kuki is Japanese-American]).
"Operation: S.P.A.N.K.E.N.S.T.E.I.N." was originally a thirty-minute Thanksgiving special, "Operation: S.P.A.N.K.S.G.I.V.I.N.G.".
Also, Lizzie's original character design was changed before her debut episode was made. Apparently her old appearance was too ugly for the execs.
One episode, "Operation: T.R.E.E.S.", an episode-long Shout-Out to Dr. Seuss and specifically to The Lorax, was axed early in its production, due to it running right on the heels of the movie version of Horton Hears a Who!.
The Toiletnator was originally going to be a one-time character whose only appearance would have been in "Operation N.O.-P.O.W.U.H."
Ben Diskin wasn't always going to voice Numbuhs 1 and 2. Numbuh 1 was going to be Tom Kenny, then an actual twelve-year-old (until he hit puberty), then Ben Diskin was chosen. Meanwhile, Numbuh 2 was going to be voiced by the same woman who did the title character of Angela Anaconda.
Originally Numbuh 4 was The Chosen One destined to defeat Dodgeball Wizard until one of the writers suggested that it should instead be his little brother Joey.