Paradoxically both the oldest and youngest

Okay, I was lying when I said Flying Battery Zone is the only level which appears out of turn in the game's internal level order. At the very end of the list, four extra acts are clumped together, making up two extra zones. In order, these are:

• Lava Reef Zone's boss act ("LRZ3")
• Hidden Palace Zone
• Death Egg Zone's final boss ("DEZ3")
• Hidden Palace Zone when accessed from a special stage ring

The placement of these stages would seem to suggest they were very late additions to the game. Indeed, if you check the sound test, song $14 is Lava Reef 2, $15 is Sky Sanctuary and $16 is Death Egg 1. It would seem Sky Sanctuary was planned to be a single act from the start, while Hidden Palace Zone ended up never getting its own song, which is kind of ironic because it did get one in Sonic 2, only for it to end up unused when the stage itself got cut.

But if Hidden Palace was a late addition to Sonic 3, the idea of naming it after the lost Sonic 2 stage might have been born even later. The name does not appear in the level select, instead being cryptically listed under "Lava Reef 4". And should you choose that stage in the prototype dated April 8th, 1994 (or 0408 prototype for short), you're greeted by this wondrous sight:

It even plays Sky Sanctuary music, which makes more sense than the Lava Reef 2 music in the final game, but isn't as fitting. After this, act 2 logically follows:

By the time the prototype dated May 17th, 1994 (or 0517 prototype for short) rolls around, Sky Sanctuary Zone is now actless, with Sonic and Tails' score tally displaying "act 1" as in the final game. Hidden Palace Zone is also now called Hidden Palace Zone, however it still plays Sky Sanctuary music.