I quickly checked the source code. And the problem seems to be that the icon image in front of the thread title is 23582 x 3888 pixels wide. It seems that Opera shows the whole image, since the size is not determined in the right place in the css.
I'm not sure how this has happened, since in each thread the link is different, but the problem remains. So it must be problem with the code that generates these images or links to them. Can't say much more than that since that is server side code…