Mario's glowing sneaker trick:
Is this really relevant to sprite 0 (vs. any other numbered sprite)? The sprite in question has
the priority bit (bit 5) set to 1.
Kinda demonstrating 2, or maybe 3 things at the same time?
1. For it to cut a hole in Mario's sprite it needed to have a lower index, so the most relevant thing here it its low index. Sprite 0 has the lowest index of all. ;P
2. Combined with the background priority bit, it cuts a hole revealing the background rather than showing on top.
3. That particular sprite is also used for a sprite 0 hit to split the scroll. Not directly related to Mario's boots, but certainly important for the appearance of the frame. Design-wise, if it didn't need to have index 0 to time the scroll split, they might have wanted to put it at the end instead so that effect 1 didn't occur?
And 4. that neither sprite 0's priority bit nor the presence of other sprites overlapping sprite 0 has any effect on sprite 0 hit detection.