Downplayed in the Film Adaptation. All the Seigaku regulars are played by the Musical's Seigaku 2nd Cast, who were all 19-22 years old at the time, while the characters were supposed to be 14-16. Hongo Kanata as well was a bit older than Ryoma. The same case is applied to the cameos in the film, since the Musical actors who played random players were around their mid-20's.
Played straight in all the Musicals, but notably averted when Ruito Aoyagi (the 1st Gakuto) was casted. He just turned 16 during one of the plays.
Shitenhouji's vice-captain Kenjirou Koishikawa was nicknamed by the Shitenhouji fans as "Banira Aisu" for his resemblance to a certain rapper.
God-Created Canon Foreigner: Ryoga Echizen was originally only created for the The Prince of Tennis - The Two Samurai: The First Game movie by Takeshi Konomi. After fan acclaim, he eventually used him in The New Prince of Tennis.
No Export for You: Neither the videogames, the rest of the anime (from episode 51 onwards, including the OVAs and The New Prince of Tennis series), the Musical DVD sets, the New Prince of Tennis manga series nor Houkago no Oujisama (" The Prince of Tennis Tribute", also known as "The Prince of Afterschool") have been released outside Japan.
Latin American viewers only had a bit better on this, since only 97 episodes were dubbed.
The official radio show of the series, Tennis no Oujisama On The Radio is officially nicknamed as "Rajipuri".
The sequel series, "The New Prince of Tennis", is abbreviated as "Shinpuri".
Screwed by the Network: The show is familiar among Toonami fans for being initially a Toonami Jetstream exclusive, and then being brought onto the television block alongside MÄR, only for both shows to be taken off almost immediately.
What Could Have Been...: Kintaro was supposed to have a larger role, and it's speculated that Konomi actually thought of having him as the main character before coming up with Ryoma.
Yuu Shirota (1st Season Tezuka 2) and Kazuki Kato (1st Season Atobe 1) played a lot with this in the backstage videos.
Daisuke Watanabe (Tezuka) and Kyosuke Hamao (Eiji), both from the 1st Season Seigaku 4th cast, started to get shipped a lot after they both starred in 4 out of 5 movies of the Takumi-kun Live-Action movies.
Any former alumni who had co-starred in a Boys Love movie with Takumi Saito (1st Season Oshitari 1). So far, we have Yoshikazu Kotani (1st Season Kawamura 2, in "Boys Love"), Hiroki Aiba (1st Season Fuji 2, in "Sukitomo") and Ryunosuke Kawai (1st Season Hiyoshi A, in the adaptation of "Itsuka no Kimi e")
"Remarkable 1st Match Fudomine" has some mixing in the cast: Since Yanagi Kotaroh was unable to perform due to his car accident, Kimeru stepped in as Ryoma, while Nagayama Takashi, who was originally playing Eiji, played Fuji. And then they called again Ichitaro Yamazaki to play Eiji.
"More Than Limit St. Rudolph Gakuen" had Kengo Ohkuchi as Tezuka (instead of Eiji Takigawa), and Eiki Kitamura as Kawamura (instead of Yoshitsugu Abe).
"Side Fudomine ~Special Match~" (2004-2005), was a remake of the original Fudomine play. This version has Endou Yuuya as Ryoma (he shared the role with Yanagi Kotaroh, who returned after his recovery). Also YOH replaced Sugawara Takuma as Tachibana, and Fujiwara Yuki played Kamio instead of Matsui Yasuyuki.
Starting with The Imperial Presence Hyotei, Date Koji was replaced by Seto Yusuke for the rest of the Musicals of the 1st Season, even when the original Hyotei Cast reunited for the special stage of The Imperial Presence Hyotei in Tokyo.
While there was a Rikkai B assembled for The Final Match Rikkai First, Masuda Toshiki replaced Yagami Ren as Yukimura, while Yamaoki Yuuki replaced Ono Kento as Yanagi, Acaba Mio as Marui and Toda Shingo as Jackal for the rest of the Season (they both played their parts for both casts). Averted for Kanesaki Kentaro, who was the only to play Sanada for both Rikkai casts during the entire season.
In Dream Live 7th, the Seigaku 1st cast made a special guest appearance, but Eiki Kitamura replaced Abe again because of scheduling conflict. It's even mentioned by the guys themselves on stage.