Got the opportunity to ride a four-star event horse the other day, he was strong through a combination and I did a little half-halt at number two - and he thought I meant to STOP. So he did. I went forward of the saddle and fetched up on his withers, but I always say it doesn't count if you don't touch the ground . . .
Riding in the rear seat is always the safest option, especially on a horse you don't know.
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.)