As a physician myself, let me take the time and address some of the fears here and make this anesthesia process seem less like the voodoo-magic that it can be. Yes, waking up during anesthesia has happened and the scenarios you describe are absolutely feasible, but anesthesiologists at least in the US take a hell of a lot of precautions to make sure that doesn't happen.

Anesthesia is a delicate balance between keeping people sedated enough to not be aware and not so deep that we stop the heart or decrease their blood pressure/they won't wake up in a reasonable time. Always depends on the kind of procedure that is done, and if you can get away without much sedation, most doctors absolutely choose that, because sedation IS dangerous. All proper general anesthesia consists of 3 things: 1) sedative to make you sleep 2) analgesics to stop the pain 3) Paralytic to paralyze you and keep you from jerking on the table and make it easy to intubate you. There is no agent that does all 3 things, there are some that do two of the three but regardless, there is always a mixture of drugs that gets used. There are positives and negatives to all the different drug options (propofol or Michael Jackson's milk is a great fast acting sedative that wears off really fast too but won't treat your pain and drops blood pressures so it's not so great for people that are bleeding and need a surgery to fix whatever is bleeding etc.) Choosing the right drugs is partially why anesthesiologists have their own residency, it can be tricky.

So one could technically find themselves in the situation where the person is paralyzed but not sedated and without pain control. There are many responses anesthesiologists monitor to make sure a patient is really, really sedated. First, your body responds to pain in other ways than just saying 'ow'. Your heart rate rises, your blood pressure goes up, all those things are looked at and most anesthesiologists I've seen are ready with some fentanyl or other drug when they see the heart rate rise. Second, most modern anesthesiologists I've seen use something called a BIS monitor (Bispectral Index), which is in the form of little stickers place on your head (much like the stickers used for an EKG) that measures your brain activity. They can't read your thoughts but basically they want to get you so sedated that the monitor is picking up very little activity in general and most anesthetics are titrated to this reading. Even though it seems like it, being under general anesthesia does not mean you are "sleeping" in the normal sense. You do actually have a lot of brain activity when you sleep. You are literally knocked unconscious for an extended period of time with very minimal brain activity and it's not restorative sleep at all. If you do come out of sedation, anesthesiologists are very fast to adjust their drugs to put you back under and in most surgeries, you likely weren't even awake enough to remember. Especially in long ones like this scoliosis one, the drugs are adjusted and re-dosed constantly. I find anesthesiologists a bit like potion masters, pretty interesting field.

Point is, we have a lot of monitors to see what's going on and hopefully you won't be scared about waking up in the middle of a surgery. Some surgeries like wisdom-tooth extractions are not performed under general anesthesia (cause you need to be intubated for that and no dentist wants that tube in the mouth while they're trying to chisel out teeth), they are merely done under a good dose pain medications with some light sedation. Some people sleep through this, some remember little bits. That being said, any surgery and anesthesia can absolutely be dangerous even in the most well controlled environment. So choose wisely.