Met normally used non-permanent fibre tip pens on OHP slides back then. Which had its disadvantages...
One day at RAF Brawdy, the Stn Cdr arrived in a jovial mood and hurled a snowball at the Met briefer "Here, Met - have some actual" he remarked. It hit the MetO on the chest and bounced onto his slides, which caused all the detail to dissolve into a purple splodge.
Not to be outdone, the MetO started his brief with the immortal words "Gentlemen, it seems that today there will be snow...".
Black flag and stack at lunchtime - not even a hint of a ten o'clock clearance!