Komischer WC3 Validator Fehler?!

<?php session_start();?>
<?php get_header(); ?>


<div class="content">
<div class="left">
    <?php if (have_posts()) : ?>
<div class="border-sep">
    <?php ot_ads('category'); ?>
<div class="title"><h1><?php wp_title(); ?></h1></div>
    <ul class="recent">
    <?php if (is_category()) {
    $this_category = get_category($cat);
    if (get_category_children($this_category->cat_ID) != "") {
    echo "<div class=\"page-children article\">
    <ul>";
    wp_list_categories('orderby=id&show_count=0&title_li=&child_of='.$this_category->cat_ID);
    echo "</ul>
    </div>"; } } ?>
    <?php query_posts($query_string . '&cat=-1'); ?>
    <?php while (have_posts()) : the_post(); ?>
        <li id="post-<?php the_ID(); ?>">
            <div class="a-box">
            <?php ot_post_gravatar(40);?>
            <a class="a-title" title="<?php the_title(); ?>" href="<?php the_permalink() ?>"><?php ot_title_limit('53') ?></a>
            <div class="info">
            <span class="valign">By <?php the_author_posts_link() ?> on <?php the_time('F jS, Y') ?> | <?php comments_popup_link('No Comments', '1 Comment', '% Comments' );?><img src="<?php bloginfo('stylesheet_directory'); ?>/img/icons/comments.gif" alt="<?php comments_number('');?> Comments" border="0" class="valign-img" />
            <?php edit_post_link( 
            'Edit', 
            ' | ', 
            '<img class="valign-img" src="'. get_option('home') .'/wp-content/themes/design/img/icons/edit.gif" alt="Edit this article" border="0" />'); ?></span>
            </div>
            <?php ot_thumb_large(); ?>
            <div class="post">
                <?php ot_the_content(); ?>
                <div class="clear"></div>
            </div>
            </div>
    </li>
    <?php endwhile; ?>
</ul>
    <?php ot_paginate();?>
<div class="adsense-category-bottom"><?php ot_ads('cat-bottom'); ?></div>
    <?php else : ?>
    <?php include 'error.php';?>
    <?php endif; ?>
</div>
</div>


<?php get_sidebar(); ?>
<?php get_footer(); ?>