Twilight Zone Episode (Help)

I'm trying to find a Twilight Zone episode that I know I've seen before. If I recall correctly it's about a group of people that go back in time and are instructed to stay on a certain path. One of them steps off the path, and kills either a butterfly or a flower; I forget; and it changes all of history. I've looked through all the descriptions of episodes and still can't find it. Can someone help????