Tails' extra sprites

If you happened to follow my suggestion of loading a savestate of the S3&K level select in Sonic & Knuckles, you'll find that you can now circumvent the restrictions imposed by the S&K alone flag, and are able to play the game as Tails.

Note that to prevent the game from crashing, Japan mode must be enabled in the S3&K savestate. This is because the game tries to load Tails' lives counter from the Sonic 3 ROM, making the Nemesis decompressor choke on the garbage data. As we previously saw though, the Miles lives counter is part of the Sonic & Knuckles ROM, so that works out.

When you do so, you might notice that Tails appears to be completely invisible. Sonic & Knuckles normally gets its Tails art from the Sonic 3 ROM, and since it's missing, the DMA transfers from that address space simply result in a string of zeroes, which translate to transparent pixels.

Knowing this, you may be surprised to learn that certain gimmicks in the later stages cause Tails to temporarily become visible again, such as the 3D pathways in Lava Reef Zone:

The reason for this lies in the Tails_Load_PLC function. When Tails' mapping frame is greater than $D1, the standard ArtUnc_Tails uncompressed art block is swapped out for the ArtUnc_Tails_Extra art block:

Tails_Load_PLC:
    moveq   #0,d0
    move.b  $22(a0),d0

Tails_Load_PLC2:
    cmp.b   ($FFFFF7DE).w,d0
    beq.s   locret_15CCE
    move.b  d0,($FFFFF7DE).w
    lea     (PLC_Tails).l,a2
    add.w   d0,d0
    adda.w  (a2,d0.w),a2
    move.w  (a2)+,d5
    subq.w  #1,d5
    bmi.s   locret_15CCE
    move.w  #$D400,d4
    move.l  #ArtUnc_Tails,d6
    cmpi.w  #$1A2,d0
    blo.s   loc_15CA6
    move.l  #ArtUnc_Tails_Extra,d6

It turns out the ArtUnc_Tails_Extra art block is actually part of the Sonic & Knuckles ROM, which is why Tails suddenly becomes visible when his mapping frame is set to a value higher than $D1. Unsurprisingly, those mapping frames don't exist in Sonic 3: they were added specifically for Sonic & Knuckles.

The implication here is that any gimmicks making use of these frames hadn't yet been designed at the time of Sonic 3's release, which is why the frames themselves are missing. On the other hand, over at Flying Battery Zone...

...Tails remains invisible in his monkey bar frames, indicating that those are read from the Sonic 3 cartridge, even when the two games are locked-on. Just another sign that Flying Battery Zone was all but meant for the first half of Sonic 3.