Mate of mine came within an ace of killing himself. He was testing an unvented gas heater that was temporarily in his main room in winter. It worked fine so since it was quite a cold day, left it running. His DiL rang him and when he didnt answer a couple of times when he was expected to, came around in person and found him unconscious. She is a senior nurse and knew what the problem was likely to be. Pure fluke that she happened to ring him then.
Took a long time to recover fully.