Sonic can hold objects, Chao included, while his Light Speed Dash gauge is powered up.

Multiple sightings of Big being spotted in the BETA version of Windy Valley from advertisements, along with the official introduction of the game, leaves us to assume Big was originally going to play in Windy Valley at one point, but was later scrapped.

The Prima Guide for SADX on GameCube has a section about raising Chao, which uses obsolete information from their Dreamcast guide. It lists abilities Chao can learn when absorbing an animal, despite they're not possible on DX. They also left in information about Chao Adventure.

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.

Officially licensed NiGHTS Chao plush released with the Sonic Adventure plush set in 1999, with it only being released in Japan via UFO Catchers.

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.

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

While Knuckles' Lost World contains the rest of Sonic's object set outside of his regular boundaries, what's also outside for an odd reason (above the boulder section) is the colored lights for the pressure plates to open the door at the end of Act 1 while never having any use.

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.

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).

Literally unplayable.

i miss my dad

When Knuckles is too far out of a map's field, he will automatically glide back to the main area. In some sections, Knuckles can get stuck gliding back by rotating into another invisible wall that would send him back. If done right, Knuckles will softlock and fly up forever.

In Casinopolis, you can enter the pinball machines whilst reading a hint monitor when using a second controller to have Tails enter the machine, of which pulls Sonic in. You can obtain a time of only a few seconds with this, as the timer won't fix itself unless you read it again.

While there's two versions of the track "Choose Your Buddy!", Finger Bass and Slap Bass version, on the OST "Sonic Adventure Digi-Log Conversation", an unused version of the song remains in the sound test of the game, it being the track without a lead guitar, never on the OST.

If you don't currently have a character unlocked, conversing with them in Adventure Fields, then obtaining an Emblem afterwards, will set a flag to have that character unlocked without needing to do so later on when you see them in the selected character's campaign.