A Sound of Thunder is a sci-fi thriller about the butterfly effect: If you could travel back in time and make one insignificant change, you would radically alter the present. In this movie the radical changes occur slowly, which has the convenience of allowing the main characters to see what is happening and attempt to fix it.
About half of the special effects in this movie look real and about half don’t, but that is good enough for me to enjoy the movie.
Personally I think that if you made an insignificant change in the past that nothing would change in the present; The effect of your change would diminish over time and be washed away by historical forces that lead to the modern world. I think that you would need to make a major change in the past to affect the present.