I just experienced anesthesia on Friday morning prior to undergoing kidney stone ultrasound lithotripsy. Usually, there's a momentary pleasant sensation as I slip under anesthesia, and I was waiting for that fleeting high as I was being wheeled into the outpatient OR, but the next thing I knew, I was waking up after the procedure was over, having never been aware of the anesthesia kicking in.
Certainly death can be like that, where one instant, everything seems perfectly normal, then that is the final awareness ever experienced, with no warning at all.
Professional wrestler Jerry Lawler described his televised heart attack in September 2012 that way, He felt fine at the announcer's table, blinked his eyes, then the next thing he knew, he was awake in a hospital bed a couple days later. All he knows of the time in between is from the footage seen by everybody else. He felt fine in the hospital before being discharged a week later, and returned to the ring after that heart attack and later also returned to the ring after a stroke in March 2018.
Maybe death isn't usually instantaneously painless, but my experience Friday morning proves it certainly can be, and definitely would have been had I never woken up.