Windy Valley went through major visual changes during the game's development. In the intro sequence, Tails and Big can be seen standing in an early prototype of this stage. In fact, Big isn't even able to access Windy Valley in the final game.

In the first Japanese release for the Dreamcast, a mechanical cowgirl billboard could be seen within Casinopolis. It would moan whenever the player touched it and was removed in international copies of the game for obvious reasons. This version however was also toned down from the Sonic Adventure Auto Demo prototype, where the girl was a fully fleshed playboy-style bunny. [Image credit: evilhamwizard]

The hidden black Chao egg found within the Egg Carrier has a unique texture in the original version for the Dreamcast. It was blue and black with pink spots. This was probably so that it would fit in better with its surroundings. This was changed in later releases of the game to solid black. Opening it will hatch a regular black Chao with seemingly no special qualities, however.

The Dreamcast disk has a PC-readable folder named "extras" that contains nine different wallpapers based on the playable characters, plus one of a Chao. Some of these pictures would later be used in the gallery in Sonic Mega Collection's Extras menu.

In the Dreamcast version of Sonic Adventure, you can explore under the deep water in the Egg Carrier Chao Garden. Under the water lies a secret ring far out of reach. This was removed in the DX update, because the depth of the water was reduced, probably to prevent Chao from swimming out of reach and getting stuck in the terrain.

Knuckles' theme song, Unknown from M.E., incorrectly states that he is a porcupine.

The song "Twinkle Cart", used when Sonic first enters the Twinkle Park stage during his story, is a remade version of Panic Puppet Act 1 from Sonic 3D Blast (from the Genesis version). The composer of both is Jun Senoue, the guitarist and keyboardist who is a key member of the band Crush 40.

The background music in the second area of Windy Valley, The Air, is an arrangement of the music from Green Grove Zone in the Sega Genesis version of Sonic 3D Blast.

Some of the textures used for E-105 ζ are actually pictures of Dreamcasts.

Cream has several cameos in Sonic Adventure DX. She flies by after Sonic and Tails clear Casinopolis, she can seen by Big in the same area, and Amy spots her flying by the train station at Station Square.

In Sonic Adventure and Sonic Adventure DX, there is an unused dragon boss with two heads.

Perfect Chaos' appearance in Sonic Generations is different to its appearance in Sonic Adventure. In that game, it was a creature made entirely out of water, whereas in Sonic Generations, it has scaly, reptile-like skin. According to an interview with Takashi Iizuka in the Prima Strategy Guide for Sonic Generations, Perfect Chaos' 2011 design is how the team had originally planned the monster; it had had to be changed in Sonic Adventure due to the technical restraints of the Dreamcast.

There are various unused voice clips, all belonging to Tikal. One of these clips are instructions on how to transform into Super Sonic. As such, it was probably a dropped feature.

Sonic Adventure is the first Sonic game to feature downloadable content, as the Dreamcast was the first ever video game console to have online capabilities out of the box. DLC ranged from celebratory holiday decorations in the Adventure Fields and a few levels, such as on Christmas and Halloween, to national ranking contests and the addition of the Chao Black Market.

An image of some staff members from Sonic Team, and a picture of a blue spined hedgehog, can be found in the game's data.

Dred Foxx, who voices Parappa in Parappa the Rapper, rapped the lyrics for "Unknown from M.E." (Knuckles' Theme).

In the 2021 Batman comic Legends of the Dark Knight #4, the front of the abandoned casino which Batman arrives to is based on the level Casinopolis from Sonic Adventure.

In the E3 1999 demo of Sonic Adventure, Knuckles had a very different voice. This was the same voice actor, but they re-recorded nearly all of his lines before the game's release. Surprisingly, voices for his entire story were included, even though the demo only had two stages for Knuckles.

The game's composer, Jun Senoue, initially went through and scrapped 8 different prototype iterations of the game's main theme song before settling on Open Your Heart.

Sonic Adventure had a limited edition release available two months before the release of the SEGA Dreamcast in North America. SEGA signed a deal with the game rental company Hollywood Video that let people rent a SEGA Dreamcast two months before the release of the console with a selection of games, one of those being Sonic Adventure Limited Edition. Although those copies were never meant to make it into the hands of the public, the copies that did make it to the public usually are incomplete or have a Hollywood Video sticker on the top of the disc.