When I first started riding I fell off the mounting block and rolled on my back with my arms out. The trainer stood there with her mouth open. She told me she had never seen anyone fall like that?
I've never fallen off a mounting block, but I've fallen off all sorts of ad hoc mounting blocks - car bumpers, buckets, tree stumps, etc. - because I HAVE to have something to climb on with and sometimes it's marginal to say the least.
One thing I finally figured out was that if I can't get up, I can get the horse down. I lead the mare into a ditch or put her below me on a steep downhill. That is actually easier (and I'm much less likely to fall off the GROUND!)