diff -ur modules/forum/forum.module sites/all/modules/custom/forum/forum.module
--- modules/forum/forum.module	2013-01-03 19:36:28.582754840 +0700
+++ sites/all/modules/custom/forum/forum.module	2013-01-03 19:37:24.090755306 +0700
@@ -578,7 +578,7 @@
     array('data' => t('Topic'), 'field' => 'n.title'),
     array('data' => t('Replies'), 'field' => 'l.comment_count'),
     array('data' => t('Created'), 'field' => 'n.created'),
-    array('data' => t('Last reply'), 'field' => 'l.last_comment_timestamp'),
+    array('data' => t('Last reply'), 'field' => 'last_comment_timestamp'),
   );
 
   $order = _forum_get_topic_order($sortby);
@@ -590,7 +590,7 @@
 
   $term = taxonomy_get_term($tid);
 
-  $sql = db_rewrite_sql("SELECT n.nid, r.tid, n.title, n.type, n.sticky, u.name, u.uid, n.created AS timestamp, n.comment AS comment_mode, l.last_comment_timestamp, IF(l.last_comment_uid != 0, cu.name, l.last_comment_name) AS last_comment_name, l.last_comment_uid, l.comment_count AS num_comments, f.tid AS forum_tid FROM {node_comment_statistics} l INNER JOIN {node} n ON n.nid = l.nid INNER JOIN {users} cu ON l.last_comment_uid = cu.uid INNER JOIN {term_node} r ON n.vid = r.vid INNER JOIN {users} u ON n.uid = u.uid INNER JOIN {forum} f ON n.vid = f.vid WHERE n.status = 1 AND r.tid = %d");
+  $sql = db_rewrite_sql("SELECT n.nid, r.tid, n.title, n.type, n.sticky, u.name, u.uid, n.created AS timestamp, n.comment AS comment_mode, IF(l.comment_count = 0 AND l.last_comment_timestamp = n.created, 0, l.last_comment_timestamp) AS last_comment_timestamp, IF(l.last_comment_uid != 0, cu.name, l.last_comment_name) AS last_comment_name, l.last_comment_uid, l.comment_count AS num_comments, f.tid AS forum_tid FROM {node_comment_statistics} l INNER JOIN {node} n ON n.nid = l.nid INNER JOIN {users} cu ON l.last_comment_uid = cu.uid INNER JOIN {term_node} r ON n.vid = r.vid INNER JOIN {users} u ON n.uid = u.uid INNER JOIN {forum} f ON n.vid = f.vid WHERE n.status = 1 AND r.tid = %d");
   $sql .= tablesort_sql($forum_topic_list_header, 'n.sticky DESC,');
   $sql .= ', n.created DESC';  // Always add a secondary sort order so that the news forum topics are on top.
 
@@ -950,10 +950,10 @@
 function _forum_get_topic_order($sortby) {
   switch ($sortby) {
     case 1:
-      return array('field' => 'l.last_comment_timestamp', 'sort' => 'desc');
+      return array('field' => 'last_comment_timestamp', 'sort' => 'desc');
       break;
     case 2:
-      return array('field' => 'l.last_comment_timestamp', 'sort' => 'asc');
+      return array('field' => 'last_comment_timestamp', 'sort' => 'asc');
       break;
     case 3:
       return array('field' => 'l.comment_count', 'sort' => 'desc');
Only in sites/all/modules/custom/forum/: forum-topic-order-last-comment-timestamp.patch