I have heard that 3% of surgical patients do not fully reach unconsiousness when they are put under. I can't imagine going in for major surgery and being aware.
I've had surgery twice where I was conscious enough to hear the doctors talking, and hear the hammer ring against the chisel as it chipped away at the bone in my leg. I couldn't feel anything because of the spinal anesthetic, but I was awake, sorta. Hearing was bad enough, feeling it would have been down right nasty.