Can we come up with singular events in history that, if something even slightly different had happened, would have changed the entire course of history?
One example would be the battle of Granicus during Alexander's invasion of he Persian Empire. There was no doubt that the Macedonian army would win, since the Persian's Greek mecenaries took almost no part in the battle at all, but during the course of the battle almost every Persian flung themselves at Alexander, hoping to kill him and stop the Macedonian invasion in its infancy. And they almost succeeded, if Cleitus hadn't saved Alexander from getting his head sliced off. There was a very real chance of Cleitus not reacting in time and Alexander been decapitated here, and if Alexander had died all of the subsequent history of the ancient world would have been radically differently. The Macedonians might still attack the Persian empire, but none of Alexander's general had any ambitions beyond perhaps conquering up to Egypt (that is assuming the Macedonian empire would have remained whole and entire). There would have been no Gaugamela, no East-West cultural synthesis, no Macedonian empire, the degree of Hellenistic influence in the East would have been markedly decreased, the West would not have inherited the concept of the 'God King' from the East and perhaps most importantly, Rome would probably have not inherited Hellenistic culture. The entire course of human events would probably have changed beyond recognition.