When DS1 was 3, we were in Sainsbury's and I'd let him sit in the trolley -- one of the smaller ones, that are quite tall. While I was getting something off a shelf, he stood up in the trolley.
I have no idea why, but I just didn't register that he was now standing up, dangerously unsupported in the tall trolley. I remember saying, "Let's get some chicken," and pushed the trolley handle to swing it towards the right shelf... And he went flying out of the trolley and landed flat on his back on the floor. Everything went in slow-motion, and I can still remember his eyes widening as the back of his head hit the ground. :(
He was screaming, I was crying, and I just held him on my lap and hugged him. I was really useless, went to pieces. Didn't know what to do.
Suddenly a woman appeared out of nowhere, took him from me, talked to him, walked him up and down and comforted him. I remember her rubbing rubbing rubbing the back of his head where he'd hit it. Eventually she gave him back to me, told me to take him to A&E for a check up, but that he'd be fine.
When we got to A&E, they couldn't find a single mark, bruise or bump on the back of his head - even though he'd fallen about 4 feet down onto the hard floor.
BUT... On the way out of Sainsbury's, something on the floor caught my eye and I stopped to pick it up. It was a tiny lead coin with an imprint of an angel on it. 
I had it cast into a silver pendant for his birthday, and it hangs up in his bedroom. We often talk about how everyone has a guardian angel, and how we met his that day. :)