This is a demo of a bug found in Chrome 18.0.1025.168. If it affects over versions (including Safari), I am unaware of this. The bug seems to affect the selector ::selection. The bug appears when ::selection:

• Has a background
• Has a inline element with changing border widths.

When the child's border width changes, it affects the background of the ::selection. Just select this paragraph and hover on and off this link to see the bug in action. As you can see, when the border-width changes back to it's normal stage, a few pixels of the ::selection's background disappears.