During the game's development, Tails wasn't initially planned to be a playable character and was only added for story purposes. Series creator Yuji Naka was asked several times if Tails was going to be playable, but gave different answers every time, ranging from "wait and see" to actually saying no. Eventually he was added because of an outcry amongst fans.

Many speculate that this is the reason Tails pilots a mech in-game. However, since he is fully controllable without it in the Chao Garden and through means of hacking (retaining his flying ability), it was most likely changed at a later time to prevent inconsistency with the play styles the game has to offer.
Contributed by Mr Knowitall
The game originally had a world ranking system that showed times and ranks from players around the world. There are also flags in the games files which were supposed to show what country the ranking was obtained in.
Contributed by Thewelshyoshi
By rotating the Analog Stick for a few moments in the Theme Select screen of the Options menu, a chime will eventually play, then a picture of the President's Secretary will appear, allowing it to be selected as a menu theme.
Contributed by CuriousUserX90
In the demo versions, the song "Escape from the City" was an instrumental. Whether the song was originally planned to have lyrics or not is unknown. This instrumental is slightly different than final version's instrumental. Part of the demo version was used in the "Classic City Escape" remix in Sonic Generations.
Contributed by Takahashi2212
Even though the game has an "E for Everyone" rating, it still has a "Mild Lyrics" warning. This is because both of the songs "Deeper" (theme of Death Chamber) and "Kick the Rock" (theme of Wild Canyon) use the word "damn". Additionally, "Kick the Rock" refers to the character Rouge as "sexy and smooth" followed by an innuendo referencing "jewels" in the next line. The parts in which both songs use the word "damn" are cut out of the loops found in the game, and can only be heard in the soundtrack versions.

The Dreamcast version doesn't contain the warning, as the Mild Lyrics warning was added to the Gamecube version, and is in the rating for every subsequent release.
Contributed by Takahashi2212
In the final game, Sonic wore SOAP shoes instead of his buckle shoes seen in previous games. However, during development, Sonic was shown to still be wearing his buckle shoes. This can be seen in early screenshots, and the E3 2000 trailer. Remnants of this can be seen in the final game. In the first cutscene of the Hero Story, when Sonic jumps out the helicopter, he is wearing the buckle shoes, but by the end, he is wearing the SOAP shoes. This error was fixed in the GameCube port.
Contributed by Takahashi2212
Throughout many cutscenes in the game, several of the characters lines will cut each other off. This is due to the length of the lines being longer in English than in Japanese, and the fact that the cutscenes weren't extended to compensate.
Contributed by Takahashi2212
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In several of Sonic's jumping animation frames, he appears to be wearing his regular shoes and not the Soap shoes.
Contributed by IcyHeroGamer
The Dreamcast version of Sonic Adventure 2 had official DLC that was available from the game's website. This DLC includes the menu themes that would later be added to the Battle edition of the game, three kart racing tracks with karts exclusive to those tracks (Sonic's special kart, the Eggrobo, and Opa-Opa from Fantasy Zone; the first two are unlockables in the Battle edition), and Halloween and Christmas-themed character skins for two-player mode.
Contributed by SonicManEXE
It's possible for Tails to drown in the Hero Chao garden. If he stands in the deep water for 20 seconds or so, a countdown will begin and he will start to drown. This does not deplete the player's lives, and only works with Tails. This does not work in the GameCube installment as the water is not as deep.
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There are leftover models and textures for a Chao Playground and a Chao Library found within the game.
Contributed by CupleaƱos
In an early trailer for the game at E3 2000, Sonic was shown in Sky Rail. However, in the final version of the game, Sky Rail is one of Shadow's stages.
Contributed by CuriousUserX90
During the final battle between Sonic & Shadow, when they use Chaos Control they'll say the quotes "I'll use your technique! Chaos Control!" and "This is the ultimate power!" respectively. But because of the fact the phrase "Chaos Control!" is a separate voice clip, it cuts off the first clip, so all the player hears is "I'll use your-- Chaos Control!" and "This is the ulltimate!--" However, Shadow never actually says "Chaos Control" so his clip is cut off by silence.
Contributed by Takahashi2212
There are also several unused lines, fully voiced by the game's English cast. Notably, some of these lines hint at scenes removed from the game's Last Story, 2 of which featured the President and his secretary, the latter (as well as Amy) telling the President not to give up hope and a scene that would take place shortly after Finalhazard's defeat, where the President says that, as a child, he admired heroes like Sonic and aspired to be as heroic and beloved as him, with the Secretary offering encouragement.

Another set of lines that would have appeared during the ending credits feature Eggman declaring he'll conquer the world without anyone's help, as well as him and Tails agreeing not to hold back next time they meet.
Contributed by MightyKombat
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During the scene where Team Dark meets with Amy, Eggman says the line, "Aaah, Amy you're timing is impeccable!" which is grammatically incorrect. The line should have read, "Aaah, Amy, your timing is impeccable!"
Contributed by ChinlessWonder
Like Sonic Adventure, Sonic Adventure 2 has hidden wallpapers that can be found by inserting the Dreamcast disc into a computer's CD drive.
Contributed by Funland47
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There is a variety of unused data in the games code. Some of which was featured in later remakes. The unused content includes art assets, text that suggests a World Ranking system was in the works, extra tutorials, and added Chao World features. Unused content present in later versions include the Black Market and unused themes (Shadow, Amy, Omochao, Maria and the Secretary).
Contributed by SonicManEXE
A number of Omochao lines were removed from Sonic Adventure 2 but were re-added as an easter egg in SA2: Battle for when Sonic or Shadow carries Omochao through the goal ring.
Contributed by PheonixGRX
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Big makes several cameo appearances in Sonic Adventure 2. He can be found in most stages and boss fights, and even some cut-scenes if you look hard enough.
Contributed by RayMonkay
According to an unused audio file for Omochao, Eggman's mustache is fake.
Contributed by DidYouKnowGaming
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In the Dreamcast version of the game, boxes in the Crazy Gadget area have green X's on them. These were changed to red X's for the Gamecube version of the game. It's believed that the change was made because the logo of Microsoft's Xbox is a green X.
Contributed by DidYouKnowGaming
Sonic Adventure 2 was sponsored by the shoe company Soap, which is why Sonic's shoes are different in this game. In addition, Soap's gimmick is that you can grind rails with them, a move that was added to Sonic's arsenal. Even after the deal expired, grinding became a series staple.
Contributed by DidYouKnowGaming
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In the City Escape stage, there's a hidden message in one of the posters. "Anti XXXX XX2 Association". Since there is a 2 at the end of the XX, and the number of letters matches up, it was thought to stand for "Anti Sony PS2 Association". At the time, the Dreamcast was being dominated by the Playstation 2 in sales.
The text was removed from later versions of the game.
Contributed by DidYouKnowGaming