The new meta.SE favicon (the "hollow bubble") looks awful on a dark background:
Note the stray light gray pixels in the corners, caused by broken transparency.
The issue appears to only affect the 16×16 pixel version of the favicon; the 32×32 pixel version embedded in the same file has correct transparency. Also, even the transparency in the 16×16 pixel version is not entirely broken; some pixels in the lower right corner (visible as dark blue in the screenshot above) do have correct partial transparency.
Anyway, since this is a monochrome icon, reconstructing the correct transparency is pretty easy. Here's a fixed version of the 16×16 px favicon (in PNG format, because imgur doesn't support ICO):