some coins may have had a flaw when minted & may have gone into circulation before it was noticed,after a retrieval of coins ,any left in circulation grow in value for collectors, a dateless 20p coin from 2008, being one, up to £100 in mint condition. The E-bay 50p might be from 2012 Olympics, commemorating swimmers, the original coin depicted a wave covering the swimmers face,these were changed to then show swimmers face completely, the 600 original ones have a price of up to £3000