An extreme example, but Scott's fate should be a good warning that even with planning, things can go wrong. Scott thought ponies were a good mode of transport and that turned out not to be the case. (For those who might not know the story, Amundsen beat Scott to the South Pole and returned home safely; Scott and his men got to the pole, but froze to death on the way back.)

Wikipedia has a good article on Scott which shows that even with planning, things can go wrong. Scott made some bad decisions, had some bad luck and paid the price for it. You may never find yourself in Scott's position, but bad things can happen, even in your local mountains. Planning may not eliminate all the risks, but it surely will minimize them.