The filling/placing of nitrogen in the place of oxygen is a problem in many critical industries. There was recently an incident at a local (large) hospital where the delivery AND maintenance staff colluded with each other to replace the theatre oxygen with nitrogen - they discussed thread types at length and STILL changed the bottles over - we're talking about a DOZEN bottles here. Not good for the person being operated on.....
I beleive the tap threads are different, but the fittings that are often ingeniously fitted to the top of bottles is open to interpretation