What was wrong with up to 200,000 20p coins issued by the Royal Mint in 2009 that made them instant collector's items?

Question

What medieval coin, first minted around 1140 by Roger II of Sicily, was adopted by Florence in 1252 and Venice in 1283, and was in use in Austria-Hungary until 1914?

Answer 1

They had no year of issue
The mistake happened when all coins from 1p to £1 were redesigned. One sold for £7,100. The last time an undated coin entered circulation was in 1672 in the reign of Charles II.