Thousands of times a day around the world, people have proceedured done to them that would be excruciating without removing conciousness. Limbs and organs are removed - and almost all wake up with no memory of the pain. We've been doing this for almost 100 years now and we're getting better at it - so good no one even asks the question of whether the persons feel pain during the event.

And yet... it seems impossible to do the same thing with the only addition bieng the person doesn't wake up the other side reliably.

How can this be so? Knock them out, then hang 'em, shoot 'em, dismember 'em or lop their heads off. Perhaps leaving it to persons who usually work in prisons is the problem here.