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), though Numbuh 5's voice actor is male in the Japanese dub.
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 5 is the same guy who played Olmec), or doing animal vocal effects. He's also voiced the Gravemind.
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).
Numbuh Three was originally only going to speak Japanese with an occasional English subtitle.
Numbuh Five originally was supposed to be The Voiceless (this was only fulfilled in "Operation: T.H.E.-F.L.Y.).
"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 in Operation N.O.-P.O.W.U.H. only.
Grandfather was originally going to have a wife shown!