Creator Backlash: CLAMP was dissatisfied on how the anime turned out to be due to Executive Meddling on the part of NHK and Bee Train. This resulted in changing the anime production staff and studios from Bee Train to its parent company Production I.G for the "Tokyo Arc" adaptation, disregarding all the filler episodes from the anime.
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 to complement their adaptation of Card Captor 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.
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.
Tsubasa-kun and Tsubasa-chan
Franchise Killer: Following the lukewarm reception of the anime's second season, Bee Train put a halt to any further Animated Adaptations of Tsubasa. It doesn't help that Bee Train became dormant since 2012 and the director Koichi Mashimo retired from the industry which leaves any continuation of the animated series uncertain.
I Knew It!: Some of the fans actually guessed the My Own Grampa thing more than two months before chapter 223 was published. Wow.
And some knew the names of the real Syaoran and Sakura a long time ago, too, although most if not all of the people who Knew It only knew one or the other
At least one person guessed that Yuuko was dead long before it was actually revealed.
Jossed: A lot of fans were convinced that Fai was going to die before the series ended. He didn't.
No Export for You: Neither "Tokyo Revelations" nor "Shunraiki" ever reached Spain, a country where Card Captor Sakura (and by extension Tsubasa) was amazingly popular. Casual viewers never learned how the Tsubasa storyline ended.
Season 2 was never released anywhere in Europe. 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 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 basically a dynastic war of distribution companies. It started when distributor Jonu Media chose to dub the Cardcaptor Sakura movie with their own VAs instead the ones which 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 liked better 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 NHK deciding to cancel the series.
CLAMP originally did a storyboard for the second season opening, but Bee Train rejected it and used a slide show in its place.
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.
CLAMP originally planned for Fai to die during the Celes arc.