I get this too when I refresh a page too often, or when I click a link several times to get it to load, but I haven't noticed whether I was logged in or not.
I have a slow broadband service and the TB forums takes longer to load than almost every other site I visit frequently so hitting refresh gets it to load a bit quicker.
For example, when browsing the basses for sale thread I usually have to hit refresh at least once or twice per page to get a page with thumbnails pics to load completely. If I don't hit refresh the page load icon spins indefinitely and all the thumbnails on the page never completely load. It's like it loses it's connection with the server after so many seconds. Occasionally I'll click a link and 10 seconds later I'm still on the same page so I'll click the link a few more times and the page will load.
I'm using FF28. I won't use 29 because they changed too many features I like. I also use fasterfox and adblock which do help quite a bit, so it's even worse without them enabled.