Chaos

People don't understand how chaotic and unpredictable the world is.

Suppose you made an absolutely perfect copy of our world, with every single subatomic particle having the exact same location and velocity.  Most people think that these two worlds would continue on the same path.  They would not.  I am quite sure that, within one minute, there would be a human being who is alive on one of the worlds and dead on another. 

The difference would come from quantum mechanical effects in the human brain.  We know that the behavior of subatomic particles is random.  If an electron moves one way instead of the other, it could mean the difference between a neuron firing and staying silent.  That one neuron could mean the difference between someone successfully avoiding an accident and getting killed in one.  There are millions of events every minute that could be pushed in one direction or another by a tiny change in the operation of someone's brain.

It doesn't even have to be a human brain.  Quantum mechanical randomness could change the action of a mosquito or flea, making the difference between someone getting a deadly disease or not being bitten.

Within a year, there would be a noticeable difference in the life of everyone on the planet.  Within a decade, the two worlds would be almost completely different.