When Eckels first sees the Time Machine, he imagines using it to escape the results of yesterday's election had the outcome been different. In entertaining the possibility, Eckels foreshadows the irony of the story's ending:
A real Time Machine […] Makes you think. If the election had gone badly yesterday, I might be here now running away from the results. Thank God Keith won. He’ll make a fine President of the United States.
By ruminating on what might have been, Eckels foreshadows the final twist, where Deutscher wins the election instead and therefore irrevocably changes the future. Eckels believes that Deutscher—the "anti-everything man"—will bring about an authoritarian regime and threaten the freedoms of the people.
Eckels's comment is also laced with situational irony because the outcome that he foreshadows is precisely the opposite of what he expected. Before Eckels leaves for the safari, Keith has won the election, but when he returns, his world has been turned upside down: Deutscher, the candidate he feared most, is the new president.
The story adds another layer of irony here: Eckels states that he "might be here now running away from the results." However, when Eckels returns to 2055 and realizes that Deutscher has won the election, he is not able to run from the results. Before Eckels can comprehend the implications of his actions, Mr. Travis kills him with "a sound of thunder."
When Eckels returns to the year 2055, he notices a subtle change in the atmosphere and finds that the English language now uses a different spelling system. After finding the culprit of such changes—a crushed butterfly on the bottom of his shoe—he realizes the irony of his situation:
“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.
After finding the butterfly, Eckels recalls Mr. Travis's explanation of the consequences of disturbing the past in even the smallest way. Eckels did not truly understand the gravity of time, but now, as he faces the aftermath of his own actions, Eckels finally grasps the damage he has inflicted on the world. Eckels is shocked that such a small and seemingly inconsequential creature as a butterfly could irrevocably change the future. The aftermath also proves the considerable power of nature over man, where a group of hunters can eliminate a Tyrannosaurus with guns but cannot escape the consequences of a butterfly's death. Not only is the size of the world-altering butterfly ironic, but also its beauty. The fact that something so beautiful could cause such destruction to the world is paradoxical and runs contrary to what readers would expect, thus making it an example of situational irony.