Harold Godwinson rushing to meet the Norman army of William the Bastard (Duke of Normandy) in October 1066. Godwinson had just won a huge victory over Danish invader Harald Hadrada, who was allied with Harold's own brother, Tostig Godwinson. Harold's army was exhausted after the victory.

When Harold heard news that William the Bastard had landed on the coast of England, he rushed forward to meet William's army, rather than regrouping his forces. Harold Godwinson's army would likely have easily beaten William's if he had simply played a bit of defense, and waited till he could've gathered more troops. Instead, Harold's exhausted army marched almost 200 miles and ended up losing a very closely fought battle at Hastings.

There are many events in history that were probably inevitable even if things had turned out slightly differently (the printing press, European colonization of America, the Protestant Reformation). The Norman conquest of England was very very far from inevitable and really almost the result of "dumb luck." William had actually intended to invade several weeks earlier, but couldn't get across the channel; if he had succeeded crossing the channel sooner, his army probably gets beat, as well. If he crosses one week later, he probably gets beat. Just by dumb luck, he had perfect timing, and Harold made a mistake that allowed him to takeover.