The Donner Party's Decision to Go

Walter Botha came to the conclusion that graphite couldn't be used to enrich uranium when in fact, the exact opposite is true. As a result, the Nazis headed off in an entirely different direction, researching an entirely different and far slower method. The entirety of modern history hinges upon this blunder because had the Nazis not changed their research focus, it is entirely possible that Hitler would have obtained an atomic weapon while America was still trying to figure out how to get past Nazi forces in North Africa.