Meeting with individual investors over the years has taught me much about investing mistakes. No matter how you classify investors (i.e. fundamental, technical or confused), the mistakes they make are almost invariably identical.
While some mistakes are the result of simply not knowing what to do, many are the results of either (i) losing interest; or (ii) getting overly greedy or fearful, particularly when the tide turns. In either case, much money is lost when people assume things will simply take care of themselves.