Sonic Adventure Factsさんのイラストまとめ

When finishing Windy Valley in Gamma's story, after the cutscene with Angel Island opening up, rather than spawning outside of Windy Valley as you should, the Japanese version (left) spawns you outside of the train station. This was fixed in future versions of the game (right).

While cutscenes in Sonic Adventure on Dreamcast are unskippable, in Sonic's Story, the cutscenes for unlocking Tails and for unlocking Amy can both be skipped by soft resetting the console (A, B, Y, X, Start); the game will continue without any issues with them both unlocked.

Various wallpapers from Sonic Adventure International's GD-Rom.

Due to the fact DX's Chao code was recycled from Sonic Adventure 2 Battle's as Battle was released first for GameCube, the game files contain leftover textures with drawings made of Dark Story characters that Chao would draw when having a high bond with any of the three.

@AppuruGumi By choosing the Chao icon and clicking the dropdown section in the center, You'll be brought to a section with Puyo Pop, Chu Chu Rocket, and the Tiny Chao Garden. Selecting it will let you choose which Chao you'd like to transfer to your GBA, which you then transfer it to SADX.

Obtaining a Tails, Amy, and Knuckles Chao are possible without hacking on the GameCube version of DX. You can obtain all 3 with the use of the "Interactive Disc Catalog Summer 2003" demo disc (Japan only), and the Tails Chao is obtainable via Phantasy Star Online: Ep I & II/Plus.

As Big, if you enter the hotel before playing Ice Cap in Adventure Mode, the entrance to the pool and Emerald Coast will be locked from an unused key set that read "なんだニれ", translating to "What is this?" Putting both keys in their slots will open up the pool.

The reason they don't respawn is because you can see them instantly jumping as you start the stage (their splash effect is seen in the water a bit before it vanishes)
This same effect happens in DX, except they never respawn until you beat the stage, even if you quit and replay.

Cream's textures, which to add is a duplicate file of what she uses in Sonic Heroes, does not properly load her textures on every version after the Gamecube port.

The black Chao egg on the Egg Carrier had a midnight blue top and pink polka dots on its shell, but was retextured in DX to just be plain black.