As Ray Bradbury's story "A Sound of Thunder" introduces its characters, setting, and plot and as it builds tension toward its climax, it incorporates several details that foreshadow occurrences that take place later in the story.
First, we have the sign that Eckels sees when he first approaches the Time Safari office. We might think at first that the inclusion of the text of this sign is merely for our information and to set the scene, and it does indeed do this. But it also points ahead to the end of the story when Eckels sees the sign again and it is quite different. We remember that we've seen the sign before, and we marvel at the change.
As Eckels waits for his trip to begin, he discusses the latest presidential election with the man behind the desk. A candidate by the name of Keith has won victory over the potential tyrant Deutscher. Both Eckels and the man are relieved. We wonder, perhaps, why this is important to the story, but it, too, points forward to a change that has occurred with Eckels gets back. Deutscher has won the presidency, and the man behind the desk is happy about it.
Shortly after the discussion about the election, the man behind the desk makes a rather alarming comment to Eckels. "We don't want anyone going who'll get scared and do something silly at the first sign of danger," he says. This is, of course, exactly what Eckels does later in the story, with tragic results.
When the group arrives in the past, Travis gives clear instructions. "Stay on the Path," he orders. Then he explains the potential consequences of changing anything, even the smallest element, of the past. It could be catastrophic: and indeed, later in the story, it is. Travis's warning becomes a foreshadow of what actually happens with Eckels, who steps off the Path and inadvertently changes the past. He changes the future, too, which is exactly what Travis says they do not want to do.