In the story "A Sound of Thunder," Ray Bradbury describes a world where time travel has become possible, and there are companies that sell travel packages to the past. One of those sells a hunting trip, in which people go back to the Late Cretaceous period in order to hunt a dinosaur.
The name of the hunting guide is Travis, and before leaving, he warns visitors that even the smallest of mistakes could alter their world as they know it, because a small change of events could detonate a series of changes that are impossible to predict. In the story, one of the characters steps on a butterfly, which causes enormous changes in the present. This phenomenon is often called the "butterfly effect."