Back in the early 80s (before lock out/tag out) the semiconductor facility I worked at had a fixture clean room with masks fed by house air. Unfortunately the lines were purged with nitrogen and the facilities person never switched back to air. 2 techs died, never knew they were being suffocated.
A few years ago I read an article about some inspectors that went into the hold of a ship that was full of scrap iron. The rusting iron had removed all the O2 from the air. They were unconscious in 15 seconds and died.
I heard second hand of a similar situation.
The connectors for air and nitrogen were identical; a situation begging for an incident.
The tech plugged his breathing mask into the nitrogen by mistake. He was twice dragged out of the room unconscious before the mistake was identified. No discomfort or even a suspicion of trouble was reported by the victim.