> How come - do you mean like the intro to Sonic 2 with the low priority sprite
I don't know about Sonic 2 but IIRC there is a situation that arises with sprite limiting. If the sprites are drawn in the backwards order, the wrong sprites will be cut off once you hit the draw limit. Front to back is the right rendering order on the actual system so it works.
Also, there's a more tricky situation. I hope my memory serves and I can explain it correctly:
Sprites have either high/low priority and this indicates how they are to be drawn with regards to the Scroll planes. Low priority sprites appear above Scroll B but under Scroll A (I think) and high priority sprites appear over Scroll A. But, these priorities have nothing to do with how sprites overlap themselves. The first sprite is supposed to be drawn on top of all the other ones.
This leads to an interesting situation. Assume the higher numbered sprite appears in the list before the other one:
Scroll B -> Low pri sprite #0 -> Scroll A -> Hi pri sprite #1
A back-to-front Genesis renderer would draw Scroll B, then sprite #0 (because it's low priority), then Scroll A, then sprite #1. But sprite #0 appears in the sprite list before #1. I think what happens is, wherever sprite #1 and #0 overlap, #0 actually shows through on top of sprite #1.
I might have gotten this wrong. It's highly dependent on how the Genesis actually draws the sprites but I clearly remember Charles describing a situation like this being used by some games. You may want to investigate Sonic 1's Marble Zone stage. When you step on the platforms in the lavas, the fireballs should appear on top, I believe (compare it with Gens and/or Genecyst to be sure.)
In a nutshell, the problem is that the sprite priority flag describes how sprites are layered with respect to scroll planes but with regards to each other, it's the order they appear in the list that matters. Hope that made some sense.