It's been published throughout the Internet — on reputable sites, nonetheless — that a sandwich changed the entire course of the 20th century (and beyond).
As the story goes, the shooting of Archduke Ferdinand that set off World War I was a complete coincidence. Gavrilo Princip was in a cafe, eating a sandwich, when he saw Ferdinand drive by. After Princip's pals failed to assassinate the archduke earlier in the day, he had the perfect opportunity to finish the job. The rest is, well, history: Ferdinand's death leads to World War I leads to Hitler leads to... well, you can follow the obvious train.
Now, one can make the case that Europe was such a powder keg in 1914 that all it needed was a spark. Even if Princip didn't assassinate Ferdinand, something would have led to World War I anyway, and history would follow is due course. We're not here to argue that.
We're here to emphasize: there was no sandwich! There is a fantastic explanation in the Smithsonian magazine (it's easily available online) which delves into the confusion. Basically, someone blew up a nonsensical rumor (that's what Internet does best!) and now it's being assumed as truth.
So, don't believe baseless rumors... about sandwiches, or about Princip being Jewish. You don't need the Smithsonian to debunk that one!