In the 2nd picture in my last post, you might have noticed the big brown square in the top-right. Strangely enough, that's a sprite that was appearing and disappearing at weird times. What you don't see in a static picture is how weird clones of the main character and enemy sprites would start appearing at semi-random intervals. Yuck.
Well, as usual, it's DS weirdness that I wasn't counting on that caused it. And this is where I launch into technical ramblings that make the eyes of the general public glaze over and start dreaming of Krispy Kremes.
See, the GBA and the DS both have this cool feature called DMA, which basically means they have a piece of hardware that copies memory from one place to another place, quickly (as opposed to the normal methods of loading a chunk of memory into the cpu, then writing it to another place in memory). Well, what I have been doing is setting up all my sprite information for a frame in regular memory, then, between screen updates, using DMA to copy all the sprite information from main memory into that magic vram that I talked about last time. (vram being the place where you tell the DS what your sprites and all other graphics should look like). But the more I debugged stuff, the more it looked like my DMA memory copy just wasn't working in this case. It worked other places on the DS, but not here. My conclusion: My DS hates me. But alas, I knew that chances were small that that was really the problem.
So after much digging (and much hatred directed at Nintendo), I found the issue: the DS has a memory cache, which, in the case of how I'm developing, is used more-or-less automatically. So, thinking I was writing to main memory, I was just writing to the cache, which hadn't been flushed back to main memory yet (the cache and memory were out of sync). The DMA doesn't trigger the cache to write back to main memory, so I was copying out-of-date information into vram. Bah. Easy fix, but like everything else in the land of debugging, took ages to figure out.
Next step: why my background is all wonky.