It's an easy fix if you care about that, in /catalog/controller/module/headermenu.php, look at lines 62 and 107.
Line 62:
Change the & (it's in red above) with &.$output .= '<a class="hide" href="' . $this->model_tool_seo_url->rewrite(HTTP_SERVER . 'index.php?route=product/category&path=' . $new_path) . '">' . $result['name'] . '</a>';
Similarly on line 107:
Change the & with &.$output .= '<a href="' . $this->model_tool_seo_url->rewrite(HTTP_SERVER . 'index.php?route=product/category&path=' . $new_path) . '">' . $result['name'] . '</a>';
The page will then be valid.