I cannot reproduce it at will either. But it is guaranteed to happen at least once a month. Usually after an intensive browsing session lasting nonstop for at least an hour, with lots of skipping from one page to another. Downloading lots of stuff may also be related. Follow a link, start a download, navigate back, follow another link... never linger anywhere for longer than two minutes. After an hour or two of that, close the browser. Go to sleep. Try to start the browser the next day. Tada!