Tell us about the time when the automation let you down.. share your disaster story if you have one

I was talking with some of my colleagues about an automation software I want to use and which they tested and end up not using. One of the arguments was that they were afraid that they could make mistakes and they will be propagated to everything.

Did you have such experiences? How bad was it, Could you share what happened? What are you doing to prevent that from happening ? (testing, validation?)