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).

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.

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.

According to an unused audio file for Omochao, Eggman's mustache is fake.

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.

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.

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.