I have been a total klutz for as long as I can remember. However, the first time that I seriously injured myself due to my klutziness happened in the mid 90's:
I had taken it upon myself to learn how to rollerblade. Seemed like a great idea at the time. I was over at a friends house, we had put on all the safety gear from the helmet to the wrist guards, the elbow guards, and the knee pads, we had just laced up our blades and were headed out and down the driveway for an afternoon of fun.
This particularly fateful trip did not last very long however, as the driveway was on a bit of an incline and the second I hit the end of the drive, my rollerblades caught the tip of the lawn and I went sprawling face first into the ground. I sat there in shock for what felt like hours but was probably only a couple of minutes, as I look down at my left arm which is bent at weird angles, and my friend ran into the house to call my mom.
She brings out the old cordless phone and hands it over (this was wayyyyy before the time when every kid and their dog has a cellphone). My mom starts asking me a couple of questions, including 'Well, how do you know it's broken?' and as I am still in shock and have not started crying yet I manage to spurt out something along the lines of, I don't think my forearm is supposed to be bent!
She quickly drives over and off we go to the hospital. It gets even more entertaining when my dad shows up, because he doesn't deal well with things like blood and broken limbs. While the doctor is trying desperately to force my bones back into place, my dad is turning multiple shades of green, and has to be escorted out of the room by the nurses. Turns out I had fractured both bones in my forearm in multiple places, and that trying to shove them back together proved to be a rather futile effort. They booked me in for surgery the next day, and I was turned into a bionic woman for a couple of years with a plate and a bunch of screws in my forearm.
Fortunately for me, most of my klutzy encounters do not involve broken limbs ;-)