Generally, it happens when the pages are continuously getting updated. For example, on Twitter and Facebook, it happens 24 hours. When your sidebar begins to reach the bottom it goes up again and you find yourself at the ststarting point again. So, you need to browse carefully and keeping your mouse browser with a good grip so that it doesn't suddenly touch the bottom end. But, it does not happen frequently here at HubPages. Only rare incidents when somebody might be posting comments or like that.