I will run this one up the flag pole, from memory, and wait for the inevitable corrections/denials.
There was a very close call in a Hunter (Hawker) sim I believe many moons ago....the story goes that bottled air/oxygen was normally used in the sim to enhance realism, unfortunately one day there was a c**k up and a cylinder of Nitrogen was installed instead of breathing gas. Soon after donning his oxygen mask the occupant very quickly keeled over and very nearly died, only saved by the quick actions of the sim instructor.
Any further input? - I heard the story 25 plus (gulp) years ago.