Slate has an article about anesthetic awareness today. "Anesthetic awareness" is when you are under general anesthesia, but accidentally awake, just unable to move. This can happen because patients are given a paralytic in addition to the thing that makes you unconscious. So, were the unconscious-making thing to fail, you might be conscious and unable to let anyone know.
It's the stuff of nightmares, obviously.
I felt sheepish asking about this, but before my surgery I did ask the anesthesiologist, "How do you know I won't be awake and just not able to say anything or move?"
I got the reassuring answer that I would not personally be receiving any paralytics, so if I were to wake up, I would in fact be able to move. This is because, since they were operating near my spine, they were going to hook electrodes up to my skull, arms, and legs, and pass signals in both directions (brain to limbs, limbs to brain) and see if they were received. There are minor risks from this procedure, he told me, but it greatly reduces the risk of spinal cord injury, because they can see right away if what they are doing is affecting your spinal cord.
I have no memories from my surgery at all. Of course, I suppose it is possible that I had some kind of experience during it and simply can't remember it, but I'm going to assume I was really out. It doesn't matter at this point, at any rate.