In my case, it's all relative to age and size:

8th birthday - my grandfather's Ithaca 12ga SxS with exposed hammers. It had a malfunctioning trigger group that most of the time fired both barrels if you pulled the front trigger - which was required to prove your manhood! Steel butt plate and cracked grip probably added to the pain. It was a family rite of passage and all the grandsons had to do it. Not sure whether the hype from my older cousins on the run-up to it or the actual shot was worse. Without an adult behind us, the shot probably would have put us on the ground. No doubt would be classed as child abuse today...