If you right-click your back button, then it displays the current cache of pages visited on this site, usually the 1st page listed on the back button is the current/last advert running on the site, which by just clicking the back button is attempting to give you that advert. The advert is usually running for a set time on the page, which is less time than that which you spend reading/answering a question here, so your browser is trying to load a "defunct" link to a page... Hence, we all get the "back button not working" scenario.
Ed does this compute with the Techies?
For a simple run-around the problem, Right-click back button>Select second page(usually page you were on). carry on as you were!
Hope this explains....:-)