My worst would have to be the time I went to ride my Aunt's lease pony. Actually, I think she was just exercising him for someone at her paddock... anyway, he was a little gray, 14.2hh and only 5 so he was very green. I was about 13/14 and virtually a beginner.
My aunt rode him first , we were in a long narrow paddock and she just rode in a circle around a tiny cross-rail, simple walk and trot stuff.
Then I got on and just walked him around for a little bit, then I asked him for a trot, and for the first few circles he was fine, then out of the blue he just took off, bolted down the paddock in a flat out gallop.
I don't remember too much, aparently he went over several jumps that I wasn't aware of, I just remember looking at the fence ahead of us thinking "He is not going to stop!" And he didn't... he just swerved at the last minute and catapulted me into the dirt.
I may have blacked out, not sure, it's all pretty hazy, I just remember that I couldn't breathe for a while as I was so badly winded, and my back hurt like hell. Once I was able to get up I had to have my hands on my back to support it and I was pretty sore for a few days.
Do not follow where the path may lead, go instead where there is no path and leave a trail...