The most re-published short story in science fiction history, Ray Bradbury's "A Sound of Thunder" deals with the idea of time travel and how subtle changes in the past can drastically affect the future. A group of people book a hunting expedition through a travel agency that takes them back in time--to kill a Tyrannosaurus Rex. They are warned to stay on a designated path from which they must not stray; any change made by a visitor from the past may cause untold changes upon their return. (The T-Rex they plan to kill has already been determined to have died, so his death--repeated over and over upon return trips--will cause no changes, the guides assure.)
There are several examples of irony in the story. Perhaps the most obvious is how the simple death of a butterfly millions of years in the past can totally alter the future from which the travellers come. Because of the butterfly's death, a different--and more dangerous--President has been elected; the language has changed; and the town appears different. Also ironic is how the man who accidentally kills the butterfly is punished in a like manner. A final irony is the title, which refers to both the sound of the T-Rex and the gunshot heard in the end.