In the Dreamcast version for Sonic Adventure, with skill and a little luck, you can get into Sonic's starting level as Tails. It is not supposed to be accessable to Tails, as Sonic is supposed to rescue him in that level.
First of course, you must get Tails as a playable character. Then, get on him and walk over to the glass doorway that leads to the pool. Jump up and use propellor flight. You should go through the roof for a bit. Now, this parts tricky: Fly through the buildings (they are only texture; you can fly through them.) and try to land behind the rail intended to block you from entering. After a few tries, you should make it.
You will be able to play the same version of the level that you do as sonic. The funny thing is, you can actually successfully play most of the level without difficulties. However, no matter what, you will die in the second part of the level, as you spawn in the wrong location.