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!)
Just about all my falls have been mounting related. Once in the saddle I feel safe:')