On a forumng discussion. If a user has manually mark posts unread set to true then views the history of a post the following error occurs
#0 /mod/forumng/forum_post.php(546): forum_post->require_children()
#1 /mod/forumng/forum_post.php(2094): forum_post->get_previous_unread()
#2 /mod/forumng/history.php(66): forum_post->display(true, Array)
#3
OU bug 11280