Sometimes, for a lot of reason, happens that has_post_thumbnail returns true also if there’s no thumbnail image. This could happen for an upgrade from a really old version of wordpress or if you have used some crappy plugin to autogenerate post thumbnail.
In this case, this plugin (in thumbnail mode) tries to show an image that doesn’t exist (but has a post thumbnail id).
to fix that, i’ve simply added another check in if ( has_post_thumbnail()) in template_thumbnail.php file on row 45:
$url = wp_get_attachment_url( get_post_thumbnail_id($post->ID) );
if ( has_post_thumbnail() && $url>"")