In the cutscene before the Metal Harbor stage in the Hero Story, Amy frees Sonic from a prison cell covered in chalk writing. She asks Sonic if he is responsible for it, but he runs off without giving an answer. Spoiler:The cutscene at the start of the Last Story reveals that the writings are those of Dr. Gerald Robotnik, and that the prison cell Sonic was held in was the same one where Robotnik was executed by firing squad after recording his video diary, featuring the same chalk writing in the background. It's unknown if G.U.N. would have also executed Sonic in the same place and manner.

The City Escape level music ("Escape from the City") actually has two versions that play during the level that sound nearly identical. The first one is the one that plays when the mission starts and features a 10 second instrumental opening and a radio directive for GUN units to capture Sonic just before the lyrics kick in, while the second version plays after Sonic gets an item in the stage and the background music restarts. The latter version is distinguished by the singer adding a "WOOOO! OH YEAH!!" vocalization at the 10 second startup of the song before the regular lyrics start and the song continues normally.

During the Crazy Gadget stage in the Hero Story, if Sonic enters the room with the colorful gravity block puzzle after 4 minutes have passed since the start of the level, Dr. Eggman will threaten Sonic through the ARK's intercom a second time. However, making it to the room before 4 minutes have passed will not trigger this dialogue.

At the start of the Dark Story when Dr. Eggman releases Shadow, a holographic message can be seen on the edge of the pit where Shadow was held that reads:


This message could be an early or unrealized hint at the existence of the Black Arms, the antagonists of the spin-off game Shadow the Hedgehog, or something similar that went unused in Sonic Adventure 2.

There is anti-piracy protection within the Dreamcast version of the game. If the console detects that the game is running from a copied disc and not an official GD-ROM, Eggman will be locked in death cycle at the start of his section in Cannon's Core. He'll immediately start falling through the floor, dying and respawning in the same position repeatedly. It's still possible to reach the end of the level, but visual glitches and a fixed improper camera angle will hinder the player's attempts to navigate the level.

During the cut-scene where Sonic, Tails, and Amy discuss Eggman blasting a hole in the moon, there is a sign in the background which references NiGHTS from another SEGA series, Nights into dreams.

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.

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.

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.

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, and the full demo version would later be used for Golden Bay tracks in the mobile games Sonic Dash and Sonic Forces: Speed Battle.

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.

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.

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.

In several of Sonic's jumping animation frames, he appears to be wearing his regular shoes and not the Soap shoes.

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.

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.

There are leftover models and textures for a Chao Playground and a Chao Library found within the game.

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.

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.

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.