Creator Backlash: CLAMP was very dissatisfied on how the anime turned out to be due to Executive Meddling by NHK. This resulted in changing the anime production staff and studios from Bee Train to Production I.G for the "Tokyo Arc" adaptation, disregarding all the filler episodes from the anime.
Executive Meddling: NHK produced the Tsubasa Reservoir Chronicle anime to be on par of their adaptation of Card Captor Sakura, but it became more lighter in comparison, since they also rejected in adapting the violent scenes of the series. To give a better insight as well, Bee Train was given sixth months to animate everything they could of the series.
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.
Actually, not at all wrong. The original Fai did die.
No Export for You: Both "Tokyo Revelations" and "Shunraiki" never reached Spain, a country where Card Captor Sakura (and by extension Tsubasa) were amazingly popular. Casual fans never learned how the Tsubasa storyline ended.
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 Spanish dubs of the movie and the anime were 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.
CLAMP originally did a storyboard for the second season opening, but it was rejected by Bee Train, who preferred to do a slide show instead.
The anime could have had a third season if NHK continued the production of the anime. However, CLAMP was dissapointed in how the anime was becoming lighter and kid-friendlier 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 more closer to their manga, even if it meant to release the anime as OVAs.