According to this article a batch of coins have been minted that contain no date. The error is thought to have affected between 50,000 and 200,000 of 20p coins and is the first time in 300 years that coins have been minted without a date. The date on the 20p coin was moved last year from the tails side of the coin to the heads side. The coins with no date were created accidentally by pairing up the new tails side with the old heads side meaning they had no date on them at all. If you are lucky enough to find any they could be worth as much as £50, quite a bit more that the face value.