IE (Internet Explorer 6.x, on Windows XP, without sp2) has a messy bug when rendering CSS based borders.
Not always, not every site.
But I saw it on my website enough times to confirm that it wasn’t a random occurance.
Notice, the image on the left … the border is straight and the text slowly keeps shifting to the right as we go down the page. But in the image on the right (which I captured after I simply refreshed the page) … it is nice and straight down – parallel to the border/line.