Marmaduke discovered this on OS X Snow Leopard. Not very reproducible so far.
Snippet from mail:
Ionel expedience the same issue on his machine (windows 7 screen resolution 1920 x 1080)
It's strange.... I do not have this problem (Mac OS), neither Bogdan (Fedora), nor Calin (Windows XP).
All Codemart team has the latest browser versions for their OS (both Chrome and FF).
Ionel did some CSS "debugging" and in some place there is a left-alignement of 21 px (I do not remember exactly the CSS property, nor the CSS file where he did the temporary change, and now Ionel is on holiday). As he changed that attribute to 20px, the white line disappeared. Maybe this helps you... if not, just ignore my suggestion, and keep the bug in mind only