A Sound of Thunder
In the year 2055, some men run a time travel company taking tourists back in time to hunt dinosaurs and other animals in the past.  They are careful to only kill animals that are about to die anyway and not to change anything in the past because they know that could change the present.  The day after a United States presidential election, they take customers back in time to kill a Tyrannosaurus rex. One customer is careless and steps on a butterfly, killing it.  When they return to the present, they find that the outcome of the election the day before has changed—a dictator was elected President of the United States.