I've heard this more recently that it's a characteristic which means that people who think they're getting better will then suddenly have a turn for the worse, because they don't feel as sick but their blood oxygen level is actually quite low -- they try to exert themselves, go upstairs say, and then faint.

Happened that way to a close friend of mine.

And this can also happen if there's a liquid nitrogen or liquid helium spill in a place without sufficient ventilation. As happened to an acquaintance of mine. If that happens get out even if you don't feel any shortness of breath.