* AudienceParticipation: The crowds cheering for Sonic before his Super transformation was actually a recording of fans during a promotion for the game's release.
* CreatorBacklash[=/=]OldShame: Jon St. Jon, Big the Cat's original voice, has so much regret and hatred for voicing Big that he purposefully forgot how to do the voice.
* DevelopmentHell: This wasn't the first attempt to bring Sonic to (non-isometric) 3D platforming. ''Sonic X-treme'' for the Sega Saturn was supposed to do this, but turmoil within the company over the game's direction reduced it to VaporWare. According to ''Sonic'' co-creator Yuji Naka in an interview for the first issue of ''Official Dreamcast Magazine'', [=SA1=] was at one time supposed to be a Saturn game, but the idea was scrapped when the company decided to focus on the Dreamcast.
** As stated above, ''Sonic Adventure'' was originally in developement for the Sega Saturn, with the Sonic World featured in ''VideoGame/SonicJam'' serving as prototype. As such, there are some elements from ''Jam'' that can be found in ''Adventure'' such as [[https://www.youtube.com/watch?v=wtSAIyXJUdE some of Sonic's animations]] and an unused spring that can be found via hacking.
** According to some DummiedOut voice clips from Tikal, Super Sonic was going to be playable in the same manner as the Genesis games (collecting 50 rings will let you transform during a level).
** The in-game engine appearance of Perfect Chaos was supposed to match how he looked in the game's FMV intro and the mural found in the Lost World level, but graphical limitations prevented this from happening. When the fight was remade for the 20th anniversary game ''VideoGame/SonicGenerations'', he was given the intended design since technology had improved enough to allow for it.
** It would appear that early on in development, the characters (or at least Sonic) would've retained the designs they had in the Genesis games. If one looks closely in Sky Chase Act 2, Sonic's model resembles his classic design (though during the cutscene that plays halfway through it it switches to the model used everywhere else). Then there's [[http://25.media.tumblr.com/37d262d21b7e85fe4228fdae2d8af264/tumblr_msrc888IiD1rfz5e1o4_250.gif this scrapped tutorial image from the Dreamcast version]].
** A very early Dreamcast prototype, called the "[=AutoDemo=]", has been unearthed and searched through. 3 test maps and many early maps were found, but the biggest find was the map geometry of the early Windy Hill, as seen in the game's earliest promotional materials. The map, along with structures in the test maps, suggested that the game would have been more dependent on Sonic's momentum, rather than his raw speed, and may have featured physics similar to the Genesis games.
** Data inside the final game refers to unused levels such as a desert, jungle and mushroom stage.
** There was a promotional image of Sky Chase with a fire-breathing mechanical dragon boss, but it's not present in the final product. [[DummiedOut It actually still exists within some versions of the game]], but when hacked in all it does is follow the player without attacking, lacks proper animations, and nothing happens if you shoot it.
** E-102 Gamma was originally going to [[DrivenToSuicide shoot himself in the head]] following his battle with E-101 [=MkII=], but this was taken out due to Sega believing that the scene was too dark for a children's game.