Seen here at phpbb.com

Currently, GD has a topic which has both a reported post and a topic waiting for approval.
I expected the forum index to show there is something needing attention, but this time there is none.

This is the topic: http://www.phpbb.com/community/viewtopic.php?f=6&t=1798975
Also note, if someone posts a topic in moderation queue there, it will display an icon in forum index.
To reproduce this, you might need to approve/disapprove other topics first.