OK, starting to gain some insight here.
What you have discovered is a very subtle issue in Unity's sprite renderer. A couple of things to notice: it doesn't happen if you scale the sprite up at all; and whether it happens depends critically on the sub-pixel position of the sprite. For example, one that shows the effect at y = 505 may not show it at y = 504.9 or y = 505.1.
I'm using just the standard Unity sprite material, with the "Pixel Snap" option off. The Pixel Snap option is not well documented, but experimentally, when I try it, it makes the issue go away. So far I haven't seen any negative effects of having Pixel Snap on, but I need to do more testing. In principle, though, this is a sensible option for me to use, since everything else in Mini Micro is set up to draw crisp pixel-style graphics, with no anti-aliasing.
It's still a bit of a mystery... textures are loaded with wrapMode set to "clamp", which means it should be impossible for the color on one side of a sprite to wrap around to the other side, pixel snap or no. So I do think this is a Unity bug at some level. But if we can avoid it with PixelSnap, I guess that's good enough for us.
Here's my test result with PixelSnap on:
and the code:
text.row = 25
images = 
sprites = 
for f in file.children
if f[-4:] != ".png" then continue
s = new Sprite
s.image = images[-1]
s.x = 500
s.y = 30 + text.row * 25