I know what you mean, but this doen't happen everytime. As for the sceenshot, If you try to see to the border of the live thumbnail (inside the glass region), you can see a line with a little shadow surrounding the thumbnail. Do you see it now? I managed to reproduce it. Just hover over the active programs in the taskbar a few times, a weird shadow effect will appear from time to time on the program "thumbnail previews". EDIT, and no, a printscreen won't capture it.