This has been happening ever since WB support for Windows 7 came out, but I don't think anyone has ever reported it. When viewing explorer in Tiles mode, there are little 'tails' coming off the progress bar bottom border:


This occurs in all UIS1 and UIS2 skins, but not in UIS0 skins. It also occurs in the explorer pane information area, which evidently uses the same images.
Hope these can be fixed...