Forum Certain threads are not reachable
 

I am trying to go through all the errors in Google Webmaster Tools and am just finding out that a huge number of threads in the forum are not reachable via their usual URLs.

Example: http://www.battleforums.com/‎forums/2002-2004-archives/1599-donut-death.html‎

What's very weird is that you can search Google for ("donut death" "battle forums"), and it will take you to that thread, and the URL is correct!! But if you enter the same URL into your address bar, you are taken to the forum index.

I would think this is a .htaccess problem, but this only happens on some threads and not others. Where do I even begin to troubleshoot this??