I was listening to a radio version of Dad's Army recently, which questioned how Captain Mainwaring had actually become a 'Captain', and it reminded me of my own father (who slightly resembled Mainwaring).
Father didn't really talk much about his activities during the war, but I think he was in The Home Guard, and was also a Special Constable. However, towards the end of the war he (who had studied Accountancy to a certain level) was commandeered to find the source of some quite serious fraud that was going on at a couple of nearby Army transit camps.
When he was interviewed for the job, the (Surrey Light Infantry?) officer amusingly asked him what rank he thought he should be given; Private or Captain? Father suggested that being a public school man, it would give him more kudos if he was a Captain; so 'Captain' he became.
I have no idea if this was ever actually made official, or written down anywhere; I suspect NOT, as he didn't wear a uniform, but when he visited the camps to inspect the books, he was apparently told to introduce himself as Captain 'Magnon'.
He very quickly found where the fraud was taking place, and a couple of Officers were eventually court marshalled. His job was done, and father returned to being plain Mister again.
He had been a quasi-Captain for about two weeks (?). My father told me this story just a few months before he died, otherwise I would never have known. I do vaguely remember him telling me about his role in fraud-busting, but not the bit about his rank.
p.s. I have always found his story a bit baffling, but just recently I learned that BBC War Correspondents were also given the honorary rank of Captain, so I'm now convinced that his story was absolutely true. He seemed to find the episode quite comical, but maybe that was the whiskey colouring his tale.