Bertrand Russell said, “The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt.” In 1999, Cornell researchers Justin Kruger and David Dunning did experiments which showed the philosopher to be correct. Their conclusion was that some people rate themselves as qualified because their incompetence is so great that it prevents them from knowing that they are wrong.
Some politicians, doctors and co-workers spring to mind. There is a good article about this at Wikipedia, see http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect.