20p Worth £50

Matthew Dent designed the new set of coins for the Royal Mint, called All Change for 2009. (Get the pun? Groan.) They're now in circulation and a whoopsie has been spotted: a batch of 20p coins - between 50,000 and 200,000 - have been minted without a date. The date on the new coins has been moved from the tail to the head and, somewhere along the line at the Royal Mint in Llantrisant, an old dateless head on the 20p was given a new dateless tail. Coin collectors reckon they could be worth up to £50, depending on how many were minted and are found (title link). So, start checking new 20p's. Update: one sold on eBay for £5,800!