Post #x is not the link to the post itself.
Clicking on it opens a JavaScript window where you can copy the actual link:
Weeelll jeez, I've always "middle clicked" the link to open it to new tab in Opera.
But this doesn't explain why the link sometimes ends with #-char and sometimes not.
Let's see, your post link copied from the JS popup:
http://www.hydrogenaudio.org/forums/index.php?s=&showtopic=64879&view=findpost&p=578866
Open it in new tab and the link changes to:
http://www.hydrogenaudio.org/forums/index.php?showtopic=64879&st=0&p=578866&#entry578866
And the actual link address is (this time with # char):
http://www.hydrogenaudio.org/forums/index.php?showtopic=64879#
Why the link is not like this?:
http://www.hydrogenaudio.org/forums/index.php?showtopic=64879#entry578866
OT: New forum bug: string &# changes into ? when "Preview Post" is clicked.