Did the ads change recently? At first, I thought it was happening because I upgraded my browser, but I saw it happening on an older version, too.
Whenever a page with ads is visited (cached or new), a series of redirects occur in the ad iframes. If I use the back button, the redirects still happen, clearing my foward history. I have to go backwards multiple steps just to visit the previous page because of the redirects (usually 2 redirects occur).
Turning off JavaScript helps, but that kills the ads entirely, removing revenue from the site. Disabling iframes doesn't seem to help because Opera still downloads/executes disabled iframes.