Many years ago on a B727 flight from KIAD to KSLC with a FAA Inspector on the jump seat....setting the picture here. The Captain noticed that the crew O2 was at zero, The gauge is actually on the FE upper panel as I recall. The Captain remained quiet as he mulled over his options. Just about the ame time the FAA guy decided he needed to use the lav and excused himself from the flight deck. A quick check dtermined that the culpirt in the debacle was the F/O's regulator which had been turned on somehow. Quick thinking the crew turned on the regulator at the FAA's jump seat and of course turned off the F/O's position. A few minutes after the FAA inspector returned to the flight deck the S/O verbalized the zero O2 condition wherein the Captain announced "everyone check their regulators". Well obviously the FAA guy was aghast to find he had drained the O2 system, The Capt. quickly asked him if they should land at Denver, to which the FAA said words to the effect, "Oh please don't do that" as I'm sure he could see his future as a FAA inspector going down the drain in a nano second. Don't recall if they descended to a lower or not.
True story and since most if not all involved haved passed on, I think it's worth telling.