It usually happens when there's a link from one forum thread to another. I don't know why, but the relative links are kinda screwy. If you want to, you can get around it though. I'll give you an example:
(Note: I put the URLs in italics so you can identify where they start/end more easily. Edit: I also colored them.)
From the Modding section there's a post called "[INDEX] - Conversion Projects (consolidated)" http://forums.sinsofasolarempire.com/174340
which has links to the popular conversion mods. The Stargate mod link doesn't work. The link should be http://forums.sinsofasolarempire.com/177748
. However, the forum gives you a link to http://forums.sinsofasolarempire.com/174340?forumid=443&aid=177748
. Ugh. If you look at it, though, you can see the original thread's number 174340
and the linked thread's number 177748
. If you delete the extra crud aside from the number of the thread you're going to (in this case, the 174340?forumid=443&aid=
part), it will give you the basic URL for the thread http://forums.sinsofasolarempire.com/177748
, and you can get where you want.
It's still annoying, to be sure. At least you can get there now.