To validate several points in this thread, I have personally never been killed by flipping an HST tractor, and I don't have any personal experience to relate.
However, when I purchased my most recent tractor the gentleman that delivered it saw my trailer hitch attachment sitting beside the driveway and commented to be extremely careful with it because he had recently gone to retrieve an identical tractor to mine and bring it into the dealership for repairs after the owner "walked it out from under himself on the back wheels" and that with a hydro it doesn't "feel" like you can. At the time, I thanked him for the information, but never inquired as to any more of the story. That being said, I do have a hitch frame for my 3pt hitch for moving my trailers EMPTY when I need to get one out and hook it up. Now, when doing any number of other tasks where there is a chance of taking a strain against the tractor, I choose to go with the teaching I received growing up that you never take a load against a 3-point unless you're using an implement designed to do so. I'd rather stick to the rule of thumb than try to calculate the justification for leaving the drawbar in the barn.
I tend to look at it this way: What could go wrong, what are the chances, and is there another method I can use to prevent it? Generally, I find there are simple, logical things I can do to prevent bad things from happening, whether I personally believe they can or not. Based on my personal experience with HST, I'm not sure I could flip it backwards because, like the driver said, it doesn't "feel" like it would. Based on a simple warning from a kind fella, I believe it can and has happened. If I wait until I have first-hand knowledge of it, then either myself or someone I know has quite possibly been seriously hurt. Regardless, I choose to avoid finding out for myself because I, for one, love my wife's husband.
Next time I'm at the dealership, I'll talk to the guys and see if anyone remembers the machine the delivery driver told me about and the circumstances it happened under.