It did that bug only once, so maybe it's a weird artifact from Safari's webkit. I didn't think of scrolling back and forth, maybe it would have corrected the problem...
Still, even if it's not totally related, I have a LOT of problems with the contact list shadow disappearing or drawing some kind of transparent window... I think some people experienced that problem already.
Looks like a webkit drawing bug. Ever since they started showing web page contents earlier, I've been getting intermittent errors like that, where the image temporarily draws in the wrong place and then isn't erased properly when it moves.