A wrapper (black border) includes some block anchors with hover, and a float with a bottom margin. The wrapper has hasLayout so in IE7-/Win it encloses the float (in other browsers this happens thanks to a clear added on :after.)
In IE6-7/Win the float bottom margin is not displayed at the first rendering, but moving the mouse down and up over the anchors (and out of the wrapper) makes the border to appear and disappear.