2014 Royal Mint sovereign struck with unfinished die
Although the gold sovereign ceased to be a circulating coin during World War I, the sovereign continued to be struck (with some gaps) during the entire 20th century, with production for collectors beginning in the 1970s and continuing to the present day. Today the Royal Mint strikes gold sovereigns in a variety of ways but the standard dimensions struck with an uncirculated or proof finish are the most common. Although struck to a high standard as one of the Royal Mint's flagship coins, errors occur as with any process, and one such error resulted in a small number of 2014 sovereigns being struck with a proof-finish reverse die and an uncirculated finish obverse die.
It is unclear how many of the mule sovereigns were struck but while rare, there seem to be dozens of such coins. Reportedly they were found in both uncirculated sovereign cards and proof sets (https://www.thesilverforum.com/topic/38823-2014-mule-sovereign-anyone-know-how-many-of-these-error-coins-were-minted/).