Well, I have't been posted in a while - a bit over a month, as I didn't post anything this year and it's Februari already - but here's something I wrote a while back.
Say you travel to the past. You blend into the crowd and you do whatever you were traveling through time for. However, someone turns his head to look at you. For some reason you get stuck in on this guy's mind. Perhaps he sensed that something was wrong about you being then and there. However, he carries on with his life normally.
Later the same day, the guy is in some boring and his mind wanders back to the moment the two of you met. Nothing too interesting, one would say. However, had he not been thinking of you, he would have been flirting with the beautiful woman from the company they were doing this project with: his future wife. Now, they don't marry six years later, they don't get a son together and now their son's son doesn't get to be the first girlfriend of another girl. Because of this girl's experience with relationships is entirely different, she breaks up with her boyfriend after three months instead of getting him a son. And she just so happened to be the mother of a major player in the third world war in the future you are from. Now this war plays out quite differently and the whole world has changed.
You did nothing, yet you changed the entire world.
Perhaps, you make sure you don't get noticed by anyone. Perhaps, you even avoid any and all contact with humans. Perhaps you are just a bit more lucky and only change less influential things. But still, you will have made a change by just being there. And as it happens, the chance that that change disappears is minute. It if doesn't disappear, it will propagate. Given enough time, nothing will be the same.