What events or people do you reckon among history's greatest failures? These need not be spectacular failures, merely great failures. Please note that, in order for something or someone to be a great failure, there must first be a reasonable chance for success, with appropriately significant ramifications, had things gone differently. By this standard, a battle in which one side is defeated by superior forces is not a great failure, but one in which it is defeated by an evenly matched foes, resulting in the ultimate loss of the war, would be. Also, some pathetic schlub who drinks himself to an early grave would be no great failure, unless he were within days of developing a cure for cancer, the secret of which being buried with him.
The most important feature, obviously, is the potential for wildly different ramifications.