Most mules are the result of using an incorrect obverse or reverse die, though very occasionally an incorrect collar die is used. A modern example of an incorrect collar die mule is the 2025 George Orwell 1984 £2 from the Royal Mint's Strike Your Own press.
On 20th January 2025 the Royal Mint changed the Strike Your Own coin design from the definitive flora design to the George Orwell 1984 design. It would appear that the Royal Mint staff did not update the blanks however, and a small number of planchets with the edge inscription IN SERVITO OMNIUM were struck with the George Orwell 1984 design. The expected edge inscription for this design was THERE WAS TRUTH AND THERE WAS UNTRUTH.
Australia's 1939 halfpennies were struck with with two major designs - Blakemore's token design and Kruger Gray's kangaroo design. Most other denominations had their reverses updated in 1938 but the halfpenny's new reverse came later, and even then in 1939, 4,382,000 halfpennies were struck with the old design and just 504,000 were struck with the new design, making the 1939 kangaroo halfpenny a scarce coin.
Within the strikings of the 1939 kangaroo halfpennies there are a number of varieties: the dies for the new design were produced at the Royal Mint, London but they did not correctly fit the Melbourne Mint's die presses, which necessitated some changes (p214, Briggs, Australia Copper/Bronze/Brass Halfpenny Die Types, 2016).
1939 kangaroo halfpenny reverse dies
Reverse Die
Notes
Fur on arm Deep rib-cage in front of knee Both serifs on base of Y of PENNY
Very rare
No fur on arm Shallow rib-cage in front of knee Right serif on base of Y of PENNY missing
Most common
No fur on arm Deep rib-cage in front of knee Right serif on base of Y of PENNY missing
Use for proofs
The original dies were apparently used to strike a small number of coins which were subsequently released into circulation: they are rare, but as it is a comparatively recent discovery, no doubt more will come to light. The updated design is largely the same but the details of the fur on the kangaroo's arm and the kangaroo's rib-cage are gone. Curiously, the right serif on the foot of the Y in PENNY has been removed - possibly in error unlike the other changes. The design used for the 1939 halfpenny proofs appears to represent an intermediate design as it still has the deep rib-cage of the original design - possibly is was tested but found to be unsatisfactory for long term use and so was relegated to the striking of proofs. Interestingly a proof is known with the original design but there is speculation that it was in fact struck at the Royal Mint, London (https://benchmarkcoincatalogue.com/coin/412?pg=79).
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.
In 1950 the reverse design used for New Zealand half crowns underwent some small changes with both the old and new design being used in 1950. Such a change is not unusual, with many of New Zealand's predecimal designs undergoing at least one small change.
The exact reason for the design change is not clear but it was likely to improve die life: possibly the move to cupronickel coinage in 1947 necessitated the changes due to the harder alloy. Regardless, the change is easy to spot: on the older design the designer's initials KG are close to the diamond directly above while on the newer design the designer's initials KG are far from the diamond directly above. Neither variety is rare.
In 1956 the portrait of Queen Elizabeth II used on New Zealand's coins was updated to more prominently show a shoulder strap on the Queen's clothing (p72, Pepping, New Zealand History Coined, 2017) - this change occurred across all denominations but the old design was also used in 1956 for some pennies and threepences. Although no catalogues report it, there are reports of the old design being used on a very small number of 1956 halfpennies.
Jerome Remick reported the existence of at least one piece which underwent thorough testing and was found to be genuine. He also estimated that perhaps 200 such coins may exist (p461, Remick, British Commonwealth Coins, 1971), although the whereabouts of the reported example or the existence of any other pieces is now unknown: presumably the variety not being listed in any catalogues has left it and other examples as unattributed.