I did manage to roll off Sam the first time I rode him. My friend's sister (who's probably six inches shorter than me) was on him before me, and we forgot to lengthen the stirrups.
Once I got on, I figured I was only gonna be on for a little while so the stirrups didn't matter. And then Sam started to trot.
Imagine (if you will) posting with stirrups six inches too short.
I went off forward and to the right, and my friend said it was a gorgeous fall - I landed on my upper back, rolled, and bounced upright. (She and I used to fight out at our Festival, and she credits combat training for teaching me how to fall.)
Yeah, when my daughter fell off after jumping a grid (the horse - who died a couple years ago at age 43, still full of buck - put in an extra jump for luck after the grid) she hit the ground like a bag of wet cement. That's when I sent her and her little brother to aikido classes to learn how to fall . . . it worked, she now falls very gracefully.