Beam Me Up, Scotty!: Mokona's Catch-Phrase "Mokona Modoki Modoki Doki!" is one of the most recognizable quotes of the series, but it was actually created for the anime; in the manga, she doesn't say anything remotely similar.
CLAMP was reportedly dissatisfied on their own way to weave the storyline of the manga. They acknowledged that they had been too far and had created too much of a Mind Screw.
Similarly, CLAMP was disgruntled with how the anime adaptation turned out to be due to Executive Meddling on the part of NHK and Bee Train. It reached the highest point in the Season 1 filler finale, where a spell caused dead people to revive - which nullifies all the plot of Tsubasa! The producers unmade it the next season, but the damage was already done, and CLAMP shot down the idea of continuing the anime. Even when they were proposed an OVA adaptation of the Acid Tokyo arc, they changed the anime production staff and studios from Bee Train to its parent company Production I.G and disregarded all the filler episodes from the anime.
Creator Killer: Although both the manga and its sister series XXXHO Li C were hugely successful, their Mind Screw ending put a permanent stain on CLAMP's perceived ability (or lack thereof) to make the right conclussions for their series. No CLAMP product since those two (and even older ones put on hiatus like X1999!) has avoided being object of mistrust or alarm due to the possibility that it may end in another mess.
Doing It for the Art: The Spanish dub director, Alfredo Martínez, watched X1999 full and studied the work of its voice actors in order to make the Tsubasa dub more faithful in the common characters.
Executive Meddling: NHK and Bee Train produced the Tsubasa Reservoir Chronicle anime akin to the adaptation of Cardcaptor Sakura, thus toning down the violence and not going into the darker chapters in the manga. To top it all off, NHK only gave Bee Train six months to produce the anime. The second season had a lackluster reception and the studio was changed to Production I.G for the OVA.
The names Fai gave everyone in Outo became quite popular among the fans: "Big Doggy" for Kurogane, "Little Doggy" for Syaoran, "Big Kitty" for Fai, and "Little Kitty" for Sakura.
Fai-mommy and Kuro-daddy are also canon nicknames that have taken on a second life in fandom.
Ass-Chin for Fei Wong.
"Cloney" for Syaoran's heartless clone, sometimes called C!Syaoran in contrast to R!Syaoran.
Junior or Syaoran-jr when Real!Syaoran turned out not to be a real at all, but Sakura and Syaoran Li's son taking his father's name.
For Tomb Raider fans, Sakura, due to having a similar outfit in the Tokyo arc as Lara Croft, is "Sakura Croft".
Fans refer to Tokyo in the Tokyo arc as "Acid Tokyo".
Tsubasa-kun and Tsubasa-chan
Franchise Killer: Following the lukewarm reception of the anime season, CLAMP and Bee Train put a halt to any further Animated Adaptations of Tsubasa. It doesn't help that Bee Train became dormant since 2012 and director Koichi Mashimo retired from the industry which leaves any continuation of the animated series uncertain.
He Also Did: The animation director for the movie, Kyoji Asano, would also go on to design the characters for the Attack on Titan adaptation.
A lot of fans were convinced that Fai was going to die before the series ended. He didn't.
There were actually fans who thought that the real!Syaoran/Tsubasa's parents were Sakura Kinomoto and Li Syaoran of Cardcaptor Sakura but this was jossed when Sakura Kinomoto herself appeared in the manga and gave her Star wand to her to clone!Sakura. Oh and the clones are Tsubasa's parents.
Neither "Tokyo Revelations" nor "Shunraiki" ever reached Spain, a country where Cardcaptor Sakura (and by extension Tsubasa) was amazingly popular. Casual viewers never learned how the Tsubasa storyline continued.
Season 2 was never released anywhere in Europe (except, ironically enough, Spain). Granted, most of it was filler, but if you want to see the Piffle and Lecourt world arcs animated and don't live in Japan, Spain or America, you're out of luck.
Kusanagi and Tomoyo got their dub VAs replaced in the OVAs.
Fei Wang Reed was first voiced by Randy Tallman in the dub but had to be replaced with R. Bruce Elliot for the second season, following Tallman's death.
The European Spanish dubs of the movie and the anime were respectively done by completely different casts due to a dynastic war of distribution companies. It started when distributor Jonu Media chose to dub the Cardcaptor Sakura movies with their own VAs instead of those who had voiced the anime, Arait Multimedia's voicecast, which caused a great offense to both Arait and the fandom. In response, after Jonu dubbed the Tsubasa movie and in turn lost the rights of the anime to Arait, the latter refused to use Jonu's voice actors and instead used their own, which also was the original cast from Sakura. The return of the voices was better liked by the fans, but the subsequent heat with Jonu pretty much rendered the OVAs impossible to be distributed in Spain.
Screwed by the Network: NHK made the anime adaptation more appealing to younger audiences as with Cardcaptor Sakura; for the second season, Bee Train produced filler episodes. The lukewarm reception of the series, combined with CLAMP's discontent with the anime adaptation, resulted in the series being axed altogether.
Troubled Production: NHK jump-started the animated adaptation in late 2004. But then Executive Meddling started to bring the project down. NHK felt that the manga was too Darker and Edgier for the series to be broadcast and pushed Bee Train for a recreation of a Lighter and Softer anime adaptation similar to Cardcaptor Sakura within six months of production time. Then Bee Train replaced CLAMP's storyboard for the second opening and created filler episodes for the next season. The series was eventually broadcast to a middling reception. The plans for a third season were ditched, and Production I.G decided to continue the series with the Tokyo Revelations OVAs.
CLAMP originally planned for Fai to die during the Celes arc.
CLAMP originally did a storyboard for the second season opening. However, Bee Train wasn't willing to make an offer with them and used a slideshow behind CLAMP's back.
The anime could have had a third season if NHK continued the production of the anime. However, CLAMP was disappointed in how the anime was becoming Lighter and Softer instead of what it was originally (as the manga's storyline was getting Darker and Edgier), and they decided to change the production staff and animation studios to make it closer to their manga, even if it meant to release the anime as OVAs.