What mistake does Eckels make while the men are hunting in the story "A Sound of Thunder."

Eckels made the mistake of walking off the edge of the floating path while hunting dinosaurs in the past.  

"This fool nearly killed us. But it isn't that so  much, no. It's his shoes! Look at them! He ran off the Path. That ruins us! We'll forfeit! Thousands of dollars of insurance! We guarantee no one leaves the Path. He left it."

Travis is visibly upset when he says the above line, because he knows that there are possibly catastrophic repercussions of changing the present through actions...

Eckels made the mistake of walking off the edge of the floating path while hunting dinosaurs in the past.  

"This fool nearly killed us. But it isn't that so  much, no. It's his shoes! Look at them! He ran off the Path. That ruins us! We'll forfeit! Thousands of dollars of insurance! We guarantee no one leaves the Path. He left it."

Travis is visibly upset when he says the above line, because he knows that there are possibly catastrophic repercussions of changing the present through actions in the past.  When Travis does say those lines, the reader still doesn't know if Eckels's actions changed anything in the present.  Sure enough though, the present is different because Eckels stepped off of the path.  The written language on signs is a bit different and the system of government has been changed.  

"You joking? You know very well. Deutscher, of course! Who  else? Not that fool weakling Keith. We got an iron man now, a man with guts!

Upon realizing that things have changed, Eckels checks his boot.  On the bottom of the boot is more than mud. Eckels has killed a single butterfly.  That single death caused untold repercussions to ripple through history.  

Embedded in the mud, glistening green and gold and black, was a butterfly, very beautiful and  very dead