Well here is a real life story and I'll try to make it short. The B727 was en route from Washington, Dulles to Salt Lake City. There was an FAA Air Carrier Inspector in the jump seat just aft of the Capt. When about 200 miles east on Denver the Capt. looked back at the S/O panel and noticed the O2 was at Zero. Just about that time the FAA asked to be excused to go back and use the lav. As soon as he was out of the cockpit the Capt called this O2 problem to the S/O's attention and everyone checked their O2 regulators. The F/O's regulator had been left on, thus depleting the O2, Quickly the Capt. moved the Observrs seat regulator to the On position. When the FAA returned and after few minutes the Capt. called the O2 problem out to the crew and said "everyone check their regulators", Well the FAA guys was mortified to find his regulator in the on position. Quickly the Capt. posed the question....should we divert into Denver? The FAA responded with a "please, whatever you do, don't do that". So they continued on to Salt Lake City without further incident. Not sure weather they decended or did any other precautionary planning. The poor FAA guy could probably see him self writing officail letters explaining his actions for the next five years! I can tell this story today as the Capt. has since passed away.