My first trip to the ER was when I fell out of my great-grandfather's car as a kid. I was 4-ish. I'd tried to open the passenger door to get in but couldn't, so I climbed in the drivers side and slid across the seat. Unfortunately, I'd managed to unlatch the door. Combine that with my great grandfather never making me wear my seat belt and oops. Once the car got moving, the door flew open and I fell out into the street. I remember rolling and then seeing my great grandfather's car stopping about 50 feet away. I have no recollection of actually falling out of the car or hitting the ground. I ended up with a huge lump on my forehead and numerous scrapes, but otherwise I was okay. I really don't remember much about the hospital visit other than them giving me a sucker, though.
Second visit I was 7. Jumped off a slide, hit a support beam on the way and snapped my femur in two. Also the only time I've ever ridden in an ambulance (knocks on wood). I don't remember much about the ER since I was pretty high on Nitrous Oxide at that point. Though I do remember yelling at the nurse who was cutting my pants off so they could examine my leg because the pants were brand new and we didn't have much money because my dad had been recently laid off.
Third visit, and only one in my adult life, was when I got my eyebrow split open by a broken practice sword. Bleeding everywhere as head wounds do. That was doubly fun since I work at the hospital and the nurse on duty recognized me. Bad enough trying to explain to a strange medical practicioner that you got your gaping head wound sword fighting, let alone one at your place of employment.