Penicillin shot when I was three, back in the day when doctors would actually inject penicillin without worrying that you might croak from an allergic reaction. Don't know what it was for, but pretty sure it wasn't for VD at that age lol. I remember cuz it was injected into my butt, and hurt like hell!
Also, just a little older, I remember when I climbed up on the sink in our apartment bathroom, and found my dad's Gillette "Blue Blades" in their slide out dispenser. Having no idea what they were, I managed to get one out, started to play with it, and was surprised to see all that red stuff (blood) on my hands (although I really don't remember feeling any pain). Yelled for mom, but have no memory of what she did or what else happened after that, apart from remembering that she was pretty freaked out...