The short story "A Sound of Thunder" has a huge element of surprise at the end of the story.
In the beginning of the piece, the reader learns of a guided safari company that allows people to travel back in time to hunt dinosaurs and other animals that are currently extinct; however, this is not without planning. Travis, the guide, states that they only choose animals that are about to die because
"we don't want to change the Future. We don't belong here in the Past. The government doesn't like us here. We have to pay big graft to keep our franchise. A Time Machine is finicky business. Not knowing it, we might kill an important animal, a small bird, a roach, a flower even, thus destroying an important link in a growing species."
He continues by explaining that even killing a small rodent could inadvertently change history, and therefore, the future.
The reader also learns, almost in passing, that a major election has just taken place, and, luckily, a man named Keith won: "We're lucky. If Deutscher had gotten in, we'd have the worst kind of dictatorship. There's an anti everything man for you, a militarist, anti-Christ, anti-human, anti-intellectual. People called us up, you know, joking but not joking. Said if Deutscher became President they wanted to go live in 1492."
Once the safari takes place, Eckels succumbs to his fear and runs off the path provided by the company that protects the integrity of the environment of the past. Travis, one of the safari leaders, is livid and even threatens to leave him in the past. Travis knows that it's possible history has changed. Lesperance, the other safari leader (whose name means "hope"), is hopeful that nothing has changed.
Once they return back to the present, it's immediately clear that something has changed, and they quickly learn that Deutscher has now won the election. The actual surprise is then revealed by saving an important detail until the very end, until after the characters and the readers see that a change has definitely occurred:
Eckels felt himself fall into a chair. He fumbled crazily at the thick slime on his boots. He held up a clod of dirt, trembling, "No, it can't be. Not a little thing like that. No!"
Embedded in the mud, glistening green and gold and black, was a butterfly, very beautiful and very dead.
"Not a little thing like that! Not a butterfly!" cried Eckels.
It fell to the floor, an exquisite thing, a small thing that could upset balances and knock down a line of small dominoes and then big dominoes and then gigantic dominoes, all down the years across Time. Eckels' mind whirled. It couldn't change things. Killing one butterfly couldn't be that important! Could it?
By saving this detail until the end and keeping it from the readers and the characters, the surprise is created.