A butterfly effect is the phenomeon that a small change in initial conditions that leads to a significantly different result than if the initial conditions haven't been changed. Have you personally witnessed such phenomeon?

Example: Your pet seeks attention and you decided to spend a few seconds to pet your pet before leaving for work/a party/shopping for grocerries/etc. You drive to an intersection and the light turns red just a few seconds before you arrive, the car ahead of you passed the intersection sucessfully. When you eventually arrive at the next intersection, you find the car ahead of you was hit on the side by another car from the intersecting road. The driver in the car ahead of you was instantly killed.

You've just been saved by your pet.