I've got a very rare 20p coin accidentally minted as a 50p coin.
Would you like to buy it off me for fifty quid?
(rnuk http://flickr.com/rnuk, Tue 7 Jul 2009, 12:23, archived)

I'll buy two!

( Theoban What of it, Tue 7 Jul 2009, 12:24, archived)