What was the big consequence of Eckels' action?

Eckels does retrieve the bullets from the Tyrannosaurus Rex, but he does go off the path. Travis had warned him repeatedly not to go off the path. Even the slightest disturbance in the past could potentially have huge complications in the future. Travis gives a lengthy description about how killing one mouse could affect billions of that mouse's descendants as well as the descendants of those animals (including humans) who are part of the food...

Eckels does retrieve the bullets from the Tyrannosaurus Rex, but he does go off the path. Travis had warned him repeatedly not to go off the path. Even the slightest disturbance in the past could potentially have huge complications in the future. Travis gives a lengthy description about how killing one mouse could affect billions of that mouse's descendants as well as the descendants of those animals (including humans) who are part of the food chain. So, even stepping in the mud and killing a mosquito could have drastic consequences. 

Upon their return, Eckels' learns that he had stepped on and killed a butterfly and brought it back from the past. It had been encased in the mud on his shoe. This single action led to significant changes. First, he notes that the English language is different. The sign about the safari is still there but the words are spelled differently. Keith, who had won the election before Eckels killed the butterfly, has now lost. This leaves Deutscher as the winner and this is tragic because he is described as a tyrant. This is the biggest or most obvious consequence of Eckels' mistake. But just consider the change in the language as evidenced by the sign. There's no telling what kind of social and historical changes resulted from and/or led to these changes in the English language. Language is shaped by culture, wars, other social aspects. The author leaves the reader wondering what other consequences Eckels' might have triggered. His mistake also leads to his death. The last thing he hears is the sound of Travis (presumably) shooting him.