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).
Before becoming a ROOT ninja, Sai was bald and had a British accent.
Kuki is voiced by Lauren Tom, who plays Amy and her mom, Inez, on Futurama, Connie (Kahn, Jr.) and her mom, Minh, on King of the Hill. And in continuing with being vocally typecast as an East Asiannote Chinese, Japanese, Korean, Laotian, etc girl and her mother in cartoons, Tom plays both Kuki Sanban and her mother, Genki in this series.
Numbuh Four is voiced by Dee Bradley Baker in one of the rare times that Baker isn't voicing an animal (Klaus the goldfish from American Dad!), doing announcer work on a game show (like Shop Til You Drop and Legends of the Hidden Templenote yes, Numbuh 4 is the same guy who played Olmec), or doing animal vocal effects. He's also voiced the Gravemind. As well as Ra's al Ghul from Batman: Arkham City.
Not to mention that in "Operation: W.H.I.T.E.H.O.U.S.E.," Numbuh One's future son is Greg Cipes, a.k.a. Beast Boy!
Stickybeard is voiced by none other but Mark Hamill (Luke Skywalker, The Joker on the 1990s Batman cartoon, the Larry 3000note interesting fact: Time Squad was ending its run around the time KND came on, etc).
In "Operation: C.H.A.D.", Numbuh 274's parents have the same voice actors as Dexter's parents from Dexter's Laboratory.
274/Chad himself is voiced by Jason Harris, host of Nickelodeon's short lived revival of Double Dare, Double Dare 2000.
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.
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]).
"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 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."