I've noticed that sometimes when I click on a link within a post that takes you to a different thread, whilst it goes to that thread, for some reason it also appears to sign me out. I get the 'Username' & 'Password' box come up and if I try to post in that thread, it won't allow me to as I'm not signed in. However, if I close my browser (or the tab that page is in) and load the site again, I am still logged in as if nothing happened.
This doesn't happen for all internal links though, so I am not sure what is causing it.
This is happening on Win XP with both IE7 and Opera, but not on Opera on OS X.
I've deleted my cookies/history/cache, etc.