Some 15 or so years ago, when I flew intercontinentally quite a lot, mostly on European carriers (KLM; SwisAir (R.I.P.) and Lufthansa with occasional Thai or SIA), they used the same 'double mono' jacks for (free) headphones. On one flight the damn thing was miswired so that I could hear right channel of program #n in left ear, and left channel from program #n+1 in right ear. Video was typically projected on a screen from a 3-CRT projector; on SIA's 747-400, on upper deck business class, the screen was blocking entry to the loo: one had to duck, almost crawl under it .