* MilestoneCelebration: Coincides with the 10th anniversary of the original game. As such, collecting 180 emblems will [[spoiler: unlock a 3D version of Green Hill Zone.]]
* NamesTheSame: ''Sonic Adventure 2'' has a level called Sand Ocean, which shares its name with a series of courses from the ''VideoGame/FZero'' franchise. Became HilariousInHindsight when Sega helped Nintendo develop ''F-Zero GX'' a few years later, which led to further deliberate [[ShoutOut Shout-Outs]] to ''F-Zero'' in ''VideoGame/ShadowTheHedgehog''.
* NewbieBoom: For many people, the [=GameCube=] ''Battle'' port was their introduction to the ''Franchise/SonicTheHedgehog'' series proper, due to being the first ''Sonic'' game to appear on a Nintendo console.
** Knuckles is voiced by Scott Dreier in this game, when he was voiced by Michael [=McGaharn=] in the first. However, their voices are similar enough, so it's hard to notice the change.
** Tails, who was voiced by Corey Bringas in the first game, is now played by Corey's younger brother, Connor. Ditto for his Japanese voice from Kazuki Hayashi-to-Atsuki Murata.
* SequelFirst: ''Sonic Adventure 2 Battle'' was released first on the UsefulNotes/NintendoGameCube a year before ''VideoGame/SonicAdventure DX Director's Cut'' was released.
** There was once a widespread rumor that Chao apparently couldn't reincarnate more than two times. It's possible for them to reincarnate potentially 255 times due to data limitations.
** While it didn't directly spawn the once-common rumor of a "Sonic and Knuckles RPG", there was some speculation that said (ultimately theoretical) project had been retooled into this game.
* WriterRevolt: The director of ''Sonic Adventure 2'' was so angry at the negative reception for Big the Cat in the previous game that he took personal offense to it and put Big the Cat as a hidden secret in every stage as a spiteful act of vengeance towards everyone. Ironically, fans are amused by his cameos (even those that hated Big) so much that they brought most of the cameos back in the PC version.
** Originally the game was only going to feature Sonic, Knuckles, and Eggman as playable characters. The alternate stages would be branching, a feature that would return in ''VideoGame/ShadowTheHedgehog''.
** An earlier premise of the story was Shadow and Rouge was from another dimension and destroying the world and it would be up to Sonic to stop them.
** Shadow’s original name was “Terios”, which means “reflection of” (but in promotional material his name was Dark Sonic)
*** Rouge’s original name was “Nails”.
** [[http://www.sonicretro.org/wp-content/uploads/2016/06/Shadow-concept-art.png Early designs of Shadow]] show him missing an eye, and at one point, he had black eyes with a singular red pupil, Possibly scrapped due to being too scary or impossible to render on his face believably.
** Sonic was originally going to wear his sneakers from the Genesis games, instead of his SOAP shoes which enabled him to grind. The demo released along with ''Phantasy Star Online'' still has these shoes (likely before the endorsement deal had been fully inked).
** The Chao Gardens are rife with unused assets, particularly in Battle. As a result of the HD releases lacking access to the GBA-exclusive Tiny Chao Garden, more of these things are unavailable in them.
*** While the Smart Fruit was available in the Dreamcast version, it is one of the three foods unavailable in Battle. It raises the hidden Intelligence and Luck stats of a Chao, while the Grapes, which raise Stamina and Luck, have a broken model that renders only the texture of the stem; and the Mint Candy provides insane amounts of stat experience but causes erratic stat increases upon levelling up that don't often match the amount given by a Chao's grade for a particular stat. The Peace and Energy Fruits from the Dreamcast version and most of the Dreamcast version tree seeds, all exclusive to Chao Adventure 2, were completely removed from Battle.
*** Two unused shades of blue, dubbed dark blue and powder blue by fans, are present within Battle's data. They come in both monotone and twotone varieties, but appear to lack egg textures. In regards to textures, Battle made the Moon texture (exclusive to the game's now defunct website which allowed you to purchase .vmi files of special Chao colors from the Black Market page when using the Dreamcast's browser) unavailable while adding four new and also unused Chao textures - Glass, a gray glasslike gradient, two dull blue gradient textures called Metal in their files, and the texture for the Pearl Race medal from Sonic Adventure. The Dreamcast version also has unused metal textures, but these appear to differ from Battle's. Additionally, the old jewel textures used in the Dreamcast version are located in a separate folder.
*** In the Dreamcast version, the Kid's Troops Chao Rangers use modified versions of all of the jewel textures sans silver, in which the textures and gleams are darker than usual. These variants can be obtained through hacking. In Battle, they just use the shiny colors instead.
*** A partially-programmed pacifier was a headgear that appears to have been supposed to place a pacifier over a Chao's mouth when given to them. However, like all headgears it just replaces a Chao's head model(s) with that of the pacifier and creates a weird effect in which the Chao appears to be headless.
*** Every Chao color except for the shiny jewel ones has an egg texture, even the colors exclusive to breeding like color twotones - an egg created as a result of breeding normally generates with the normal egg texture. They all have dummy data that allows them to be purchased from the Black Market in egg or shell form should they be hacked in.
*** As mentioned above, the HD rereleases rendered the jewel colours and Tiny Chao Garden-exclusive fruits unavailable. In addition, the texture system was not properly ported and renders both the usual shiny twotone and shiny jewel colors unavailable as a result. Bred shiny twotones render with ridiculously bright textures with little of the original colors visible.
*** [[http://www.sonic-cult.org/dispart.php?catid=3&gameid=5&subid=2&artid=5 Some leftover data]] from the Dreamcast version shows that originally the there was a Chao Playground and Chao Library planned. The Library would allow you to create new files for the Chao Adventure VMU game, but the Playground's function is unknown. Geometry data for them still exists in the PC version.
*** The Hero and Dark Gardens would have had three toys each like the normal Chao Garden if the pianos were implemented. It's likely that they would have been given as prizes for winning part of the Hero and Dark Races, but unknown if said races were originally going to have more than four levels. The Hero Garden piano would have a Chao play a piano rendition of the Chao Garden theme or VideoGame/SonicAdvance[='s=] multiplayer waiting theme, while the Dark Garden organ would produce an organ rendition of the Tiny Chao Garden music.
*** For a long time, it was believed that the Knuckles and Amy Chao were unavailable legally and that the Tails Chao was only available through a special mission in VideoGame/PhantasyStarOnline - prior to Battle the Knuckes Chao was an UrbanLegendOfZelda. In reality, all three official character Chao are available on the Gamecube version of Battle through an obscure Japanese demo disc released in 2003 - the Tails Chao is technically Neutral, the Knuckles Chao is Hero and the Amy Chao is Dark. All three are unobtainable in the HD versions.
** In ''Battle'', after getting an A-Rank on [[spoiler:Green Hill Zone, Super Shadow and Super Sonic]] would have been unlocked in two-player mode. All that exists are the unlock message and character select descriptions, Although its possible that Metal Sonic uses the unused code for both of them.
** The game's programmed to spawn a Goal Point in stages meant to be completed by Knuckles or Rouge should you hack the game so that you could play as the other characters within those stages, suggesting that there was going to be a sort of "free play" mode where you can play as any character in any stage.