When combining border radius and a box shadow, the rounded border is seemingly anti-aliased against black. See screenshot.
Created attachment 16222 [details]
The CSS, to reproduce:
-webkit-box-shadow: 0px 0 20px rgba(0, 100, 255, 0.8);
Created attachment 16223 [details]
More egregious in the second example:
-webkit-box-shadow: 10px 10px 10px rgba(0, 100, 255, 0.8);
By the nature of antialiasing, the black fill used to cast the drop shadow is showing through the antialiased clip that's supposed to cover it. There are similar issues with solid borders, e.g.
<div style="width: 100px; height: 100px; -webkit-border-radius: 20px; border: 10px solid white; background: red;"></div>
I think some common cases involving solid backgrounds/borders can be fixed and sped up. The cases reported here are more tricky because the background is transparent.
These cases, but not the one mentioned in comment #5, were fixed in <http://trac.webkit.org/changeset/32660>.