When I visit a forum index, go into a thread from there, then return to the forum index by pressing the 'back' button, the page displayed is not the one I viewed just before going into the thread - instead it's an archived version of the page that is somewhat older.
FYI, I use the 'back' button on all forums I go to, and this is the only forum I know of that does this. (so it can't be my browser or anything like that)