I was a repairman for a gas utility for a number of years, and investigated hundreds of complaints of carbon monoxide hazards.
The most people I ever packed off to a hospital from one call was eight. That was a restaurant where the manager had turned off the ventilator over the range to prevent a lot of heated air from being exhausted outside and cooling off the restaurant.
Unfortunately, several burners on the range were dirty and making carbon monoxide. People in the kitchen were being poisoned as a result.
I've seen people get poisoned off one dirty pilot light on a gas range.
Since the specific gravity of carbon monoxide is almost exactly that of air, carbon monoxide tends to hang around for hours, rather than dispersing. People who warm up their cars on a cold day, even outdoors, can create enough CO inside or against a house to set off CO alarms two or three hours later.
Personally, I don't have a CO alarm, because my ability to evaluate equipment and hazards makes the alarm unnecessary, and I find the false positives annoying.
After a recent storm here that left 500,000 people without electrical power, often for several days, a number of people were killed by carbon monoxide. Most commonly these were immigrants using charcoal indoors, but also people using electric generators outdoors where the exhaust gasses infiltrated the house.
In short, while rare, these problems can be real, and can be fatal.