Just happened again, this time the 'latest replies, ‘popular threads’ and ‘connect with us’ parts were added at the bottom of the page, making it even longer. As I said earlier, it’s not the same every time 
I am always navigating to this forum from a phpbb forum, although I doubt that makes a difference.
edit: and again, this time the entire upper bar with banners etc was also missing. Just the splash damage logo remained, all the menus were in text below eachother.
http://www.splashdamage.com/forums/showthread.php/31850-ET-Legacy-(compatibility-project)
This thread is empty for me, no posts :s but still unread.
The homepage breaking up upon first visit is still happening regularly 
I haven’t got any, but I’ll install Chrome and use that for a while.
Forum update happend? 
On the " There are no threads newer than the previous one. Click the back button in your browser to return to it. " page, there is an empty combobox onthe bottom left with no slectable options :s
And the sidebar on the main page does not stay closed anymore?
Yeah side bar keeps re-appearing for me. Close it, leave the forum and come back, it’s there again.
No Problem
And the emtpy combobox appears on each page (leftunder). It has “Quick Style Chooser” in it 
When somebody bumps an old thread in which I made a post, I suddenly become subscripted to that thread and receive a notification accordingl. However, my profile settings are set to “Do not subscribe”.
Didn’t any of the other old-timers notice this yet?
I noticed it today with the April Fool’s thread about T.R.A.M.S. started two years ago.
Does this happen for every thread you’ve posted in or just old ones? AFAIK the default setting is to subscribe to threads you’ve replied to, so if you changed that at any point, it’ll only affect threads from that point forward.
I didn’t post that much past year, but I guess it only occurs in old threads.
I never explicitly enabled thread subscription, but as you say some update to the default settings might have brought this upon me. I have it disabled now, I can’t recall when I changed it.