Worst Mistake Ever

What's the worst mistake you've ever made? How did it change you?
I was able to recall that sinking feeling in my last novel at a critical moment. Your neurons fire like crazy to catch up with reality. There's that chill and stomach flop that, even at ten, makes you say, "Oh sh*t." My worst case happened at my friend's house in Alaska. On a military base, all the kids played war. We'd climb up on the car port roof, the dumpsters, and trees. It didn't matter who won; rather, the most spectacular death and best surprise ambush meant the most.
I found an air gun in my friend's room. It was heavier than normal, but available. Mine was still on the porch. He was watching cartoons, not paying attention--sucker. So I snuck up on him. Just before I fired his gun, I called his name. Fortunately, he turned slowly. When I pulled the trigger, it didn't sound right. I had never seen a head wound before. Blood was everywhere. In a blind panic, I got his father immediately.
Then it got weird. While applying direct pressure, the dad reamed him out for leaving the BB gun pumped and loaded. If he had turned faster, he might have (yes it's a cliche) lost an eye. This was the model that could go half way through a phonebook if you pumped it enough. In all the commotion, nobody bothered to tell my father. But that moment of dread when I heard the gun fire will be burned into my brain for the rest of my life. I will never be a hunter. I'm an engineer who pesters his wife to use safety precautions if there's a one percent chance of someone getting hurt.
What was your worst oh bleep moment? Do your characters have one?