This is a bit bothersome. I have to scroll the page horizontally sometimes to read a thread or post.
My suggestion is to extend the frame for the forum so that it fits properly. The side menu could be moved a bit more to the right to make room for this. Also I suspect if done right it should remain centered, so in fact the forum frame would be expanded so that it retains it's centering in the browser along with the top bar and side bar.
Or the forum lay out could be fixed to fit in the frame properly.
I noticed not all sections of the forum suffer from this. I guess it depends on if a user posted a image or youtube video. Try and make it so at the least, youtube videos fit properly. And perhaps make rules telling users to use thumbnail links instead of posting large images as this also causes the rest of the thread to go off the side of the frame and then would require horizontal scrolling which I know is not fun for some people. :P