Apparently some geezer has etched a drawing of Jane Austen on only four of the new £5 notes, and if you find one, you may be able to sell it for £20,000. I'd have thought this was an illegal act, defacing the currency. I've checked mine, but no luck so far.
(I could however etch my own picture, of Trump perhaps, or Ed Balls dancing. I doubt anyone would give me £20, never mind £20,000 though.)