"George Braly will be the first to tell you: he’s lucky to be alive. Braly runs Tornado Alley, an aircraft retrofit company based in Ada, Okla. Tornado Alley develops, tests, markets and installs aircraft modifications. It was aboard one of his modified aircraft, while conducting a test flight, that Braly almost lost his life because of a kinked oxygen line.
“On the test flight, I needed to get above 18,000 feet,” Braly told AIN’s The Human Factor: Tales from the Flight Deck. “I was headed out on a round-robin flight plan over western Oklahoma that would take me to between 24,000 and 25,000 feet.” Braly is no novice when it comes to flying small airplanes into the flight levels. Between 1968 and 1981 he logged 4,500 hours at high altitudes aboard a turbocharged, unpressurized Cessna twin.
But none of that experience prepared him for the moment when, alone in the Cirrus SR22, he lost consciousness. In adjusting his seat, Braly had apparently rolled over his oxygen line, stopping the flow of O2 to his mask. As he drifted off into unconsciousness, the aircraft continued on autopilot to fly along its programmed course.
Twenty minutes later, Braly said he vaguely heard the voice that had been calling him for 15 minutes. That voice, he said with certainty, saved his life. “The next thing I remember, a very nice lady was calling my aircraft number in an urgent and anxious voice,” he recalled. “I heard that while I was still not fully conscious, but it roused me.” Braly was able to descend below 10,000 feet, recover from his brush with hypoxia and eventually land the airplane. The controller had been anxious, persistent, even aggressive, Braly remembered, and he has no doubt she saved his life."
https://www.ainonline.com/aviation-news ... compliance
Here is an example of a pilot experiencing hypoxia. Fortunately it was recognized and the controller got him to descend.