Index: wp-includes/default-filters.php
===================================================================
--- wp-includes/default-filters.php	(revision 3877)
+++ wp-includes/default-filters.php	(working copy)
@@ -42,6 +42,7 @@
 
 add_filter('comment_text', 'convert_chars');
 add_filter('comment_text', 'make_clickable');
+add_filter('comment_text', 'forceBalanceTags', 25);
 add_filter('comment_text', 'wpautop', 30);
 add_filter('comment_text', 'convert_smilies', 20);
 
Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 3877)
+++ wp-includes/formatting.php	(working copy)
@@ -505,6 +505,9 @@
 	return $newtext;
 }
 
+function forceBalanceTags($text) {
+	return balanceTags($text, 0, true);
+}
 
 function format_to_edit($content, $richedit = false) {
 	$content = apply_filters('format_to_edit', $content);