In the short story 'A Sound of Thunder' by Ray Bradbury we follow the story of a man named 'Eckels.' He is in a position to travel back many millennia in time to visit the prehistoric world via a time machine attraction run by associates of a Time Travel Theme Attraction company. This privilege comes at a price however, and Eckels breaks the code of conduct for visiting the past - not to touch or influence a single thing which could change the course of the future in the tiniest way. Killing a dinosaur that was about to die anyway would be acceptable as long as he never left the virtual walkway above the jungle. Eckels is told that if he becomes frightened he must proceed slowly along the walkway back to the safety of the time machine. However in his fear and panic at the gigantic ferocious stinking beast, he panics and proceeds back to the time machine at speed, so breaching the boundaries slightly. When he gets back he looks down at his boot, only to see with shock that he must have killed a butterfly as it is stuck lifeless to his foot.
When his companions get back to the time machine, they notice the dead butterfly too and are furious as they will all have to pay a huge fine and face the consequences of changing the future. He offers to pay ten times what the journey cost to make up for his blunder. His companion Travis has other ideas and orders him to go back down the path alone to get the bullets out of the dead dinosaur's mouth, because they are from the future. Eckels no doubt thinks he has done enough, but when they all get back to their own time he immediately picks up a vibe, a scent, an awareness that things have changed - and not for the better - a chilling thought.