Dummied Out: Budokai 1 has unused tournament announcer callouts for the Saibamen, Cell Junior (Who are unplayable Mooks in story mode) and Cui of all people (Who only appears in one cutscene in the Frieza saga.) To add to that, 3 has unused announcer soundbytes hinting that Zarbon, Dodoria and Android 19 were set to return, as well as a playable Bulma (A theory supported by a 3D model of Bulma appearing in one of the tutorial missions as well as a bonus DVD showing her VA Tiffany Vollmer recording fight shouts for her). Also some of the unlockable Dragon Universe sound files hints that Super 17 might have also been a playable character, or at least a story mode boss as said vocal has Goku greeting 17 in his gruff, Super Saiyan 4 voice.
Keep Circulating the Tapes: Curiously, the Budokai HD Collection is one of the select Xbox games NOT on the backwards compatibility list, meaning you're limited to the seventh generation consoles if you want to play the first and third game, with the same going for Burst Limit. The hunt can be made exponentially worse if you want the original Kenji Yamamoto soundtrack in place, as they're limited to PlayStation 2 and Gamecube copies. Budokai 2, Infinite Worlds and the Shin Budokai games get hit with this much harder, as none of them ever got remasters, Budokai 2 in particular being the most infamous exclusion from the HD Collection's lineup.
Limited Special Collector's Ultimate Edition: A version of Budokai 3 exists that allows players to select either English or Japanese voices as well as 3 extra costumes for Goku, Piccolo and Future Trunks. There was a bit of a snafu wherein vanilla Budokai 3 discs were shipped with Limited Edition packaging. This is the version included in the HD Collection.
No Dub for You: The PAL release for Budokai 1 restricts the voice acting to the Japanese cast. Likewise, Americans wouldn't get the option to switch to the Japanese cast until the special edition of Budokai 3 was released.
No Export for You: The Dragon Ball Z 2V version of Budokai 2 is Japan only and is notable for featuring Cooler as an alternate costume for Frieza. The Japanese version of the same game also had a story stage based off of Neko Majin Z as well as Kuriza being an alt costume for Frieza.
No Port For You: Budokai 3 never got a GameCube port. Seeing how Budokai 2 came out on that system around the same time 3 came out on PS2, Namco Bandai probably didn't see the point in porting it, and given the year-long release window between the original first two games and their ports, the GameCube version of Budokai 3 would've been released between late 2005 to 2006, near the end of the GameCube's lifespan.
Mayumi Sho returned as Chi-Chi for the first game. It was the first time since 1991's Cooler's Revenge that she played the character before handing the reigns to Naoko Watanabe, who has since been Sho's permanent replacement.
While they're non-playable in this series, the first game was also Bill Townsley's first outing as Guldo and Cui, the former having been previously voiced by Dylan Thompson in the anime.
Conversely, it was Mark Britten's final time voicing Burter and Korin, though the first installment was released a year after he'd parted ways with the franchise.
In Japanese, while Toku Nishio returned to play Mr. Popo in Budokai 1, his health had declined in the following year and so was forced into retirement, passing away in 2005. Dai Matsumoto replaced him in Budokai 3 and also returned in Budokai Tenkaichi 1. Yasuhiko Kawazu would permanently become Mr. Popo's voice in future media afterward.
Short Run in Peru: Each entry was released in North America and Europe within the same year, usually in the November-December time frame. The Japanese versions wouldn't be released until February of the following year.
What Could Have Been: The unlockable "Memories of" voice files in 3's Dragon Universe mode hint at stories that never made it in, such as:
A possible alternate outcome to the final fight with Kid Buu, where Vegeta would wish for Goku's tail to grow back and Goku would end up achieving Super Saiyan 4 as a result.
A possible playable Super 17, as Goku has voice files where he greets 17 using his gruff Super Saiyan 3/4 voice. Super 17 was later added to the roster in Infinite World.
A story for Kid Goku involving him traveling through time to the future and fighting Piccolo, mistaking him for his evil father.
A Saibaman surviving the fight on Earth and helping the heroes on Namek, complete with Krillin taking pity on the Mook.
A Cell Junior rebelling against Cell, claiming that knows "what Cell is hiding."
Broly attacking Future Trunks' timeline forcing him to warn Gohan in the past. This one has at least one leftover kept in, as you fight Gohan in Broly's story, Gohan mentions that Future Trunks warned him about Broly.
Bulma was once going to be playable in Budokai 3, seeing as her name is in Dummied Out lines from the World Tournament Announcer and the bonus DVD in the American collector's edition shows Tiffany Vollmer recording fighting shouts. Bulma does have a complete model, as seen in the training mode where she teaches you how to use capsules.
Miscellaneous
It should be noted that in Japan, Budokai 1 to 3 were simply known as Dragon Ball Z 1 to 3; Japan didn't use the Budokai naming until the release of the Shin Budokai games.
Curiously, the name is, perhaps accidentally, retained for each game if you look at the data on your memory card for the Nintendo GameCube version.
