Two things spring to mind:
- Striking a bicyclist with my car.
It was dark, he was wearing black and had no light. I was turning left; bicyclist was in the right lane approaching. I wouldn’t even have noticed him but that my girlfriend at the time screamed. Dude splayed across my windshield. He hit the ground and stayed there until after paramedics attended to him for about twenty minutes. Amazingly, he wasn’t badly hurt. It was in the city, on campus so there were like ten observers, all of whom dialed 911 on their cells within seconds.
I was trying to help the guy up, shaking like a fool, apologizing like a madman. One of the witness bystanders (perhaps a law student?) pulled me aside and told me I should be careful what I said. The cops were there in under two minutes. Ambulance shortly thereafter. First aid was administered. Amazingly - despite me being a 17-year-old kid, the cops took my side. Told the rider he needed to get a light on his bike, “or else.”
We exchanged info, I apologized profusely again. The bicyclist was pretty cool about the whole thing, despite some nasty bruises. I never heard another word about the incident.
Then I went to my girlfriend’s dorm room and just sat on the bed and shook for like an hour. Hurting myself, well I can deal with that. Nearly killing somebody else - that’s entirely beyond anything I ever want to experience again. To this day, ten years later, I still am overcautious about checking all possible accident routes when I drive. I suppose that’s a good thing.
- Smashing my face
I was probably only four years old. This might be my first vivid memory. I was at my grandparent’s house. I was being overactive, like four-year-olds tend to be. My grandmother had a step-stool for the kitchen. For reasons known only to four-year-olds, I decided that an excellent game would be this: climb up to the top step on the stool, then jump off. Grandma scolded me at least once - “Stop that, you’ll fall and hurt yourself”
I don’t know why, but I remember climbing onto the top step and looking through the window into the microwave. Grandma was defrosting some ground beef. For whatever reason I still have a vivid memory of that pound of ground beef turning in the microwave. Anyways, I jumped. But this time I misjudged. I overshot by a few feet. I caught the top of my open mouth directly on a chair, smashing my front teeth. I distinctly remember the force and the shock, though not the pain.
My next memory is sitting on Grandpa’s lap, sobbing and shrieking. He was patting my head and telling me it would be all right. This is memorable because he is a famously unemotional man - even at that young age, I remember understanding that if he was trying to comfort me it must be bad, because anything up to a broken bone was typically met with “walk it off.”
My next memory is being in the emergency room. My parents were holding me down while a surgeon did…I don’t know. This is one of my worst memories. I remember screaming a lot. Also the sheer horror at being restrained while somebody did painful things to me (granted, it was necessary.) That was just triage. I remember being at the dentist the next day. Again, my parents held me down while the dentist poked, pulled and prodded. I still have a distinct memory of hands covering my face to hold me down while the dentist removed shattered teeth. shudder
Unlike most pre-schoolers, I had dentures until my adult teeth came in. 
Interesting fact - I jokingly mentioned this incident to my Grandmother, twenty years after the fact. Now, my Grandma is a tough lady. Lots of kids, a couple of wars, general “Texas” attitude (where she grew up). On my mentioning, she began weeping. Never mind that I’m grown up and have forgotten the pain (though not the horror of the treatment, as a child). Never mind that I have no trace of the trauma but two bent teeth where the adult teeth were affected by the blow.
“You just kept cying, ‘I just want my teeth back’”.
Bad experience for me, terrible experience for her. I dread an experience like this if I ever have kids of my own.