The butterfly’s wings

My favorite tipping point is that of the fears of time travel. As Ray Bradbury wrote in "A sound of Thunder", he tells of a man given the opportunity to time travel. The man is given only one instruction, "do not stray from the path". Well the guy does stray from the path and he ends up killing a butterfly, no biggie. When he gets back, everything is different and it's not for the better. All because he killed a bug in prehistoric times, he changed life as he knew it.