The day after the Fukushima nuclear incident, the top-voted article on HN was an extremely confident-sounding article by an expert about how this was a minor incident, sure to be contained and forgotten. The comments in the article were dismissive about alarm. I remember feeling a sigh of relief that the experts had pronounced it a minor incident. As the days rolled by, the news about Fukushima turned from bad to worse to horrible. Unfortunately, my archive.org skills were not good enough to dig up the front page of HN around that time to show you the article in question.
A closer tale about experts being wrong, and persistently so is the tale of the Japan Bonds' "widowmaker" trade: apparently, a lot of smart people have been wrong about this:
http://www.businessinsider.com/hedge-funds-feeling-confident...