When Freeza blew up my home planet. [spoiler]Seriously though: The British order for the ANZACs to rush Turkish defensive lines at Gallipoli Napoleon and Hitler both attacking Russia in winter. Hitler starting a war on both fronts. Mardonius' decision to attack 'retreating' Greek heavy infantry at the Battle of Marathon. This attack resulted in his death at the hands of Arimnestus and the routing of his forces at the hands of the superior Greek Phalanx. [/spoiler]