In Ray Bradbury's short story "A Sound of Thunder", humans have figured out how to travel in time. There exist companies that make money from this, including the one in the story, Time Safari, Inc. The idea is that hunters can travel back in time to shoot dinosaurs. In order to make sure that what is done in the past does not affect the present, the company takes great precautions. Workers from the company travel back first, to identify dinosaurs that would die anyway in the 5 minutes or so after moment the hunters are then sent to. They have floating walkways, so that the time travelers don't interfere with any lifeforms, and even retrieve the bullets from the dead dinosaurs. They already know that things are not foolproof, as several guides and hunters have been killed, and make no guarantees for the hunters' safety. After Eckels, the main character, is terrified by the "sound of thunder" coming from the hunted T. rex, he steps off the path, and returns with a dead butterfly on the bottom of his shoe.
Things are, at first, not obviously changed, but then Eckels becomes aware that the present has, indeed, changed. Changes are small; the air smells different, and colors seem off. The clerk looks different, and his desk looks different. Signs are wildly misspelled. The main change seems to be that the results of the presidential election of the day before have changed, as has who has the clerk's support.