I once had a QH mare flip over on me. I got on and she reared numerous times, until she finally had enough and came down on top of me. Ended up at the hospital getting x-rays, but all was fine. I was very bruised, but otherwise ok.
A friend of mine was trotting her horse in a field and he tripped and fell over on top of her! It was wild- tro, trot, trot, BAM! No warning at all! She ended up in the hospital over night for monitoring a concussion. Otherwise, she was ok, too!
BTW- We were both wearing helmets, and do, everytime we ride!
Good luck with the project :)