Markup Error

I think you have an error in the markup generation:

In _inc/functions.php line 422 you should replace:
$output .= '</a><div class="pop-cl"></div></li>';
with
$output .= '</div></a><div class="pop-cl"></div></li>';

otherwise there’s a div not being closed.

Regards,
Agustín

http://wordpress.org/plugins/popular-widget/