The Royal Navy have a thing called a Defence Council Instruction... basically formal notification of changes to uniform, regulations etc.
Naval personnel are allowed to grow a full beard, but not a mustache, the other services can have a mustache, but not a beard.
In the early 80's an out of sequence DCI arrived at HMS Drake and did the rounds stating that as from midnight on 31st March this would change.
Local TV came in and filmed the "First" sailor having hs full set trimmed down to the new regulation mustache.... as he left resplendent in his facial hair (cameras still rolling) he was stopped by the Master at Ams and asked what the abomination under his nose was!
At which point the date dawned.... cruel but superb