I'm looking for a story that I think was written by Bradbury. If anyone knows the title, please let me know. It's about some people that go back in time but are told they can only use the special walkways that were built. One person discovers a butterfly on the bottom of his/her shoe and thinks that it won't have caused any harm, but when they all return to the present, the world is completely different. Does this sound familiar to anyone?

"The Sound of Thunder". A great story.