Submitted by: Jan Abaza

Open the game's [Megadrive] music test. It plays BGMs in the following order:

Green Grove Zone act 1
Green Grove Zone act 2
Rusty Ruin Zone act 1
Rusty Ruin Zone act 2
Volcano Valley Zone act 2
Volcano Valley Zone act 1
Spring Stadium Zone act 1
Spring Stadium Zone act 2
Diamond Dust Zone act 1
Diamond Dust Zone act 2
Gene Gadget Zone act 1
Gene Gadget Zone act 2
Panic Puppet Zone act 2
Final Fight Zone

It shows the early level order. It makes sense; since Diamond Dust is one of the longest levels in the game, it could easily be one of the 'conclusion' zones.
This carries over to the PC and Saturn versions in a way; this is the level order for the PC Sonic 3D as it appears from the CDA track listing:

Green Grove Zone act 1
Green Grove Zone act 2
Rusty Ruin Zone act 2
Rusty Ruin Zone act 1
Spring Stadium Zone act 1
Spring Stadium Zone act 2
Volcano Valley Zone act 2
Volcano Valley Zone act 1
Diamond Dust Zone act 1
Diamond Dust Zone act 2
Gene Gadget Zone act 1
Gene Gadget Zone act 2
Panic Puppet Zone act 1
Panic Puppet Zone Act 2

Some errors were partially fixed, but more were created. Note how Spring Stadium Act 1 and 2 have been reversed for no reason whatsoever.