Even if you don't think you edited it, it's the only reasonable source for the extra character. You should have this:
PHP Code:
$content .= cat_ul_manage(0, $prev_cat_depth, 0) . '</ul>' . "\n";
if (SHOW_CATEGORIES_BOX_SPECIALS == 'true' or SHOW_CATEGORIES_BOX_PRODUCTS_NEW == 'true' or SHOW_CATEGORIES_BOX_FEATURED_PRODUCTS == 'true' or SHOW_CATEGORIES_BOX_PRODUCTS_ALL == 'true') {
// display a separator between categories and links
if (SHOW_CATEGORIES_SEPARATOR_LINK == '1') {
$content .= '<hr id="catBoxDivider" />' . "\n";
Maybe your site is somehow misreading the "\n" after the '</ul>'?
That is only a view source formatting character and should not be displayed on the website screen.