The problem occurs only when using a vertically repeated body background to provide a fixed width site's boundaries, usually done so that a shadow/gradient effect can be used either side. When the central section is white, as in this case, a dark item intended to be full width of the site will show it up quite markedly. The problem does not occur if the background is on another element, only the body. Adding another wrapper div is quite often not an option as this means the minimum width of the site will include the shadows/gradient.
Whilst the left body padding is the usual fix, I discovered some time back that odd/even background/site widths works a bit better.