That's the thing about any CTB method, nothing is 100% guaranteed. From what I've read on SaSu, and understand that everything you read on here is pretty much anecdotal, a failed SN attempt is not supposed to leave someone in a vegetable state, or with any serious damage, should the attempt fail. But, who really knows? I don't know one way or the other. All we can do is go by what is reported. I know the method that is least likely to leave someone in a vegetative state, or worse off before they attempted CTB, is by shotgun blast through the mouth, since there's like a 99.99% chance of success with that method. Hell, even death penalty executions have been screwed up and left the condemned in worse shape, but still alive, at least for a while until they finally died. And those were the result of so-called "professionals" doing the job. The truth of the matter is that there is nothing 100% guaranteed in this world.