Creator Backlash: The designers didn't like Mystique's first season look, with the reason given for its use being that they ran out of time. They preferred her design from Season 2 onward due to being closer to what they envisioned.
Directed by Cast Member: One of the directors for the French dub was Alexandra Corréa, who was also Rogue's voice actress.
The first season's action scenes were restricted by the network, who felt that kids shows had gotten too dark and violent for the demographic they wanted to reach. After the first season was a success, they were allowed a lot more leeway. Boyd Kirkland has spoke positively about this, saying that this allowed for the first season to properly develop the characters and their personalities.
Due to sensitivity after September 11th, cuts were made to the episode "Growing Pains". In the episode, Avalanche risks his life to save Shadowcat from debris; but because network censors were worried about children's reactions to a character having something fall down on them, the scene was cut. So at the end of the episode, Lance appears to be holding Kitty for no real reason. Frank Paur, who directed many episodes of the series, believes this censorship made viewers confused about the Lance and Kitty romance initially since it established why Kitty changed her opinion of him from Season One. All DVD releases restore this scene.
Later in the series, three of the X-Men were Put on a Bus, supposedly because the cast was too large. Wolfsbane, a character who hardly spoke, was an obvious choice. Jubilee, however, had a very minor role but did have a few subplots with Bobby and Sam, and appeared considerably more than Wolfsbane. Then later Evan decided to live in the sewers because his powers were growing out of control. The irony is that Sunspot, a character who appeared even less than Wolfsbane, stayed even though he did virtually nothing for the rest of the series, resulting in two girls being the only New Recruits dropped.
Word of God has also stated there was pressure put on the show at times to include more superheroes and characters from the wider Marvel universe. The producers though wanted to keep their own universe small and the focus on mutants rather than having to explain how The Incredible Hulk, Spider-Man or The Mighty Thor fit in. Nonetheless, compromises were evidently made; Captain America guest starred in a Season 2 episode, Nick Fury and S.H.I.E.L.D. became a recurring presence in the show's latter half, and HYDRA (led by Viper) served as the villains of two episodes in the fourth season.
This trope also led to the creation of X-23. Marvel executives wanted more Wolverine, but the show's producers felt the focus should be on the kids. X-23 was seen as a compromise since she would be a young female clone of Wolverine. Judging by the fan reaction, the X-23 example is perhaps the best proof that all Tropes Are Tools and even the "worst" can sometimes yield pleasing results.
Spyke was originally meant to be the team's tank with his powers giving him a carapace with the spike firing as backup weapons. This changed when the producer insisted that he should be more marketable and enforced the removal of the armour plates leaving him with the spikes, accidentally making him similar to Marrow. The haircut and skateboarding were also caused by this.
The general standalone nature of the first two seasons, aside from the finales, was mandated by the network. After Season 3, the writers were left to their own devices and pushed for more connected, continuity-driven stories.
Forge was going to give Kurt the Star Trek "live long and prosper" sign and Kurt would try and return it unsuccessfully at the end of MiddleVerse, but Kids' WB! didn't want the show to reference Star Trek.
Fake American: Most of the series' voice cast were Canadian.
Fake Australian: Pyro is voiced by Trevor Devall, who is Canadian.
Fake Brit: Nicole Oliver does an English accent to play Risty Wilde. Since she's actually Mystique, she counts as an in-universe example.
Less so with the show, as it is available on iTunes and YouTube (Netflix has the first season as well), but just try to find a legal copy of the two original songs from the "Walk on the Wild Side" episode.
Also the DVDs are all out of print, and the last 9 episodes have never been released to DVD at all because Warner Home Video lost the rights to the series.
Although Storm was mainly voiced by Kirsten Williamson, some of her dialogue was recorded by Colleen Wheeler (Mystique's voice actress) and wound up being heard in certain episodes. Similarly, Magneto is voiced by Christopher Judge for most of the show, but for a single episode recap, he was voiced by Michael Donovan (Sabretooth's voice actor) for no apparent reason.
Brian Drummond was originally going to voice Cyclops and even recorded his dialogue for the first episode, but the role was eventually recast to Kirby Morrow leading to to everything being re-recorded.
As Madam Hydra, Lisa Ann Beley take a break from her usual heroic roles and play a villain for a change.
Andrea Libman usually voices Genki Girls and cutie characters, but had an episode voicing the tortured X-23.
Production Posse: The show shares most of its cast with Class of the Titans, which is also about gifted teenagers undergoing superhero training.
Refitted for Sequel: Spyke was originally planned to have a carapace, but this was dropped early on by request of the producer to make the character more marketable. While the original intention was dropped (to make him more "tank-like"), Season 4 features Spyke having become more heavily armored due to his mutation.
Role Reprise: In the Brazilian dub, there are three from X-Men: The Animated Series: Fernanda Baronne as Rogue (she was the character's second voice in the 90's show), José Santa Cruz as Magneto, and Eduardo Dascar as Gambit.
Screwed by the Lawyers: The show isn't streaming yet on international versions of Disney+, due to Warner Bros. still owning distribution rights to the show outside the US. The show is available on HBO Max in Brazil.
Screwed by the Network: Apparently, the cancellation was because the Executives at Marvel didn't feel like the show was good enough to continue. Supposedly, they were never really behind the series, which annoyed the occasional show director and art designer Steven E. Gordon as a number of ideas in the show were stolen/adopted by writers at Marvel; notably, before her creators brought her in themselves, then-Marvel head Joe Quesada brought X-23 into comic canon with NYX, a series about mutants living on the street, that featured X-23 as a prostitute. Yeah, a lot of fans don't like to talk about that.
Series Hiatus: The show ended up taking a much lengthier hiatus than normal three quarters into the third season. After "Under Lock and Key" aired on November 30, 2002, it didn't resume until August 2, 2003 - a little over eight months.
Short Run in Peru: At least one season managed to air in New Zealand on WNTV long before it was screened in the States, resulting in many fanvids with 'wntv' watermarks in the corner.
Star-Making Role: Character examples. Kitty Pryde being featured as a main character led to an increase in popularity for her, bringing her to mainstream attention. Nightcrawler as well to a lesser extent (as he had been featured in the Mutant Academy video game and appeared a couple of times in the 90s series). Both characters were introduced into the live action films as a result.
The Alpha Bitch that bullies Kitty in her introduction episode is called Riley Finn, after a character from Buffy the Vampire Slayer. As has been noted before, Kitty's comics personality was a big influence on Buffy's.
Word of Gay: Mystique and Destiny, as confirmed by a few blog posts by Steven E Gordon, the art designer and occasional director.
Word of God: The creators state that Spyke was not supposed to be a male Marrow. They also brought up the Kitty/Rogue scene in Spyke Cam and said of the Buffy/Faith dance it mirrors in Bad Girls something along the lines of, "We loved that scene, that was so awesome we have to find a way to allude to it in the show."
, with Kitty taking Buffy's role.