Years ago when I was working at a stable for some people I used to know, some kids were having a riding lesson.
I'm a pretty good rider, so the fella asks me to jump on a horse and show them how to take a jump. Fair enough, on I get.
The instructor raises the fence so it's a little over a meter, easy enough. I took the jump fine, but then the horse is trotting towards some spare fences on the floor, normally a horse would just walk over them as they were spaced out, while I wasn't paying attention the thing took a jump over them and I got a whiplash. Not painful at all, unless I looked left, but I got a cool neckbrace.
Most painful thing would be later that year, I was lifting some buckets of water (I was in a massive rush because I had to take a pair of shires back to a field a few miles away) and I lifted them totally wrong and put my back out. I could not stand up straight for 4 months. That was agony.
Also, stepped on a pebble once and broke my instep. Not really painful, but painful enough to stop me using that foot until it got better.
Also when I was little, I put a staple through my penis and accidentally stapled it to the snooker table my parents bought me for my birthday. (It didn't hurt at all really, but I was a nudist as a child and was trying to fix my stapler, next thing I know my foreskin is attached to the snooker table and as if by magic all my relatives started appearing for sunday lunch. Needless to say they all gathered round to have a good laugh before helping me liberate my todger)
Worst pain I've been in has to be when my mum hit me upside the head with a huge supersoaker water pistol for giving her lip. I was about 10ish. Gave me a huge gash under my ear, an inch higher and I'd have been blind the doctor said. I was gushing blood and had to lay in the hospital being restrained by nurses while they stitched the gash up without anesthetic. I still have the scar next to my ear.