"Christmas tree" files
From the 1930s until the 1980s, the BBC kept a number of clandestine files on applicants accused by MI5 of political subversion, in particular those deemed to be communists or fellow travellers of communism, and also members of far-right organisations. They were marked with a distinctive upward-facing green arrow, which bore resemblance to a Christmas tree and had the effect of blacklisting a number of applicants for roles in the BBC. Knowledge of the files was made public in 1985 after being reported on by The Observer.