Width of #content with no sidebars overflows its parent container (#main). This is due to both #main and #content having their widths set to the same value ($zen-grid-width) while not accounting for #content having margins based on $zen-gutter-width.
There seems to be no need to set the width of #content when no sidebars are displayed.
Fix is to remove the following code from fixed-width.sccs:
* The layout when there are no sidebars.