When sprites use background art

This is something I came across while messing around with Flying Battery Zone's palette. There's this flat-colored tile which appears in the foreground with palette line 1, but also appears in the background with palette line 3. I guess the color index just happened to line up.

When I messed with the layout of palette line 1, suddenly the colors didn't line up anymore, requiring said tile to be split in two, one for the foreground and another for the background. Undeterred, I went looking for another tile I could usurp.

Well, that didn't take long. I replaced the second of the seemingly redundant tiles, and everything seemed fine until I ran into this monstrosity:

Long story short, since those collapsing platforms look a lot like the rest of the level, their tiles are stored along with the main level art. That way, they can be reused by the background planes without wasting extra VRAM. Apparently this is common practice; Hydrocity Zone has collapsing platform graphics at the start of the main level art in a similar manner.

The presence of duplicate tiles gives it away: sprites need all of their tiles to come in sequence, so the art file caters to them. Background planes aren't bound to such restrictions, so they end up using only the first tile, and as such survive my hamfisted experiments.