Thankfully breaking a bone. The only time I broke one was back in the summer of 1999 on the way out to the car with my family, I think we were headed into town for something when I slipped on our porch and fell weird. As my parents supported me down to the car, out of instinct I put weight on my broken leg.
I can still remember the loss of absolutely everything else, my mind was just flooded with that agony.