With general anesthesia nothing really, I remember them pushing the meds and a strange sensation as they did so, then the next thing I know I'm being wheeled back to recovery. All times I was still a kid, so may not be the experience an adult has.
Under twilight sedation I never go completely under and usually remember the whole thing, the last time it happened they said I had an unusually high tolerance to the medication. It was enough to keep me calm, but I was very much alert and so I just asked the surgeon to narrate what he was doing because it was honestly fascinating. All those experiences were for eye surgeries as an adult.