At Coningsby we had to provide met briefs for each trip. Over to the weather-guessers, get a brief, make up OHP slide.
One day there was a classic warm / cold frontal system across the UK, so I lovingly produced the blue and red frontal positions and black isobars on my synoptic chart all in chinagraph - a work of art by the time I'd finished...
Give brief. Place slide on OHP and switch on. Everything appears in black on the screen. Puzzled for a moment I realised that chinagraph is opaque, so my artistry was all silhouetted.
"Terribly sorry, it must be one of the old black-and-white OHPs", I announced and carried on with the brief. No-one queried my comment! Even the trip went well....for once!