In 1975, New Zealand's 20c pieces were struck by the Royal Mint, Llantrisant. Among the 5,027,000 20c pieces struck in Wales, a small number were struck on scalloped edged planchets.
In 1975 the Royal Mint, Llanstrisant was also striking coins for Hong Kong, including the first striking of the Hong Kong $2 coin, which was struck with a scalloped edge. No doubt a small number of Hong Kong $2 planchets were accidentally mixed in with New Zealand 20c planchets, leading to the striking of a small number of wrong planchet New Zealand 20c pieces. It is unknown how many were struck: one was sold in Mowbrays numismatic auction 31 in September 2023 as lot 359, but no doubt there are other examples.
Following the death of Queen Elizabeth II, the Royal Mint struck a number of commemorative coins, including silver £2 and £5 proofs. The silver £5 reverse was a young and old portrait of Queen Elizabeth II with ELIZABETH REGINA above and 1926 - 2022 below; the silver £2 had a single old portrait of Queen Elizabeth II surrounded by the legend ELIZABETH REGINA 1926 - 2022. Both coins had a portrait of King Charles III on the obverse: the £5 had the legend CHARLES III • D • G • REX • F • D • 5 POUNDS • 2022 • and the the £2 had the legend CHARLES III • D • G • REX • F • D • 2 POUNDS • 2022 •
The £2 and £5 are both the same diameter and during production, the £2 obverse die was accidentally used to strike at least one £5 - an easy mistake to make given the identical diameter and almost identical obverse legend.
The mule was apparently spotted by NGC when grading a 2022 £5 and so far only a single example is known (Rivera, Discovery of the first known Mule of the Royal Mint's commemorative coin honoring the life and legacy of the late Queen Elizabeth II in Coin News, August 2023).
In 2006 the Royal Australian Mint struck fine silver proof sets of Australia's circulating coins as well the defunct 1c and 2c pieces to commemorate 40 years of decimal currency. The coins were also struck using the anachronistic Arnold Machin portrait of Queen Elizabeth II, again to commemorate 40 years of decimal currency.
During production of the fine silver proof sets, the Royal Australian Mint appears to have accidentally struck a small number of the 2c pieces with the Ian Rank-Broadley portrait. 2c Ian Rank-Broadley dies had been prepared for the striking of bronze and gold 2006 2c pieces using the Ian Rank-Broadley portrait and presumably some of the silver coins were struck with the wrong obverse, or perhaps less likely, some of the bronze or gold proofs were struck with a silver planchet.
In general, major mint errors in Australian decimal coins are fairly uncommon, but double-headed or double-tailed coins are especially rare: the presses apparently make the production of such coins difficult, and possibly impossible in more recent years. Nevertheless, the Royal Australian Mint has been responsible for a number of double-headed and double-tailed coins over the years.
Presumably in 1974, a double-headed 1974 20c piece was struck, with the heads sides being at 180 degrees to each other. This appears to be the only such known double-headed 1974 20c piece, and it is likely that the sides being at 180 degrees to each other is because the press did not allow the dies to be inserted at the same orientation. A number of other such error coins are known from the 1970s and it is likely the result of mint workers experimenting in their spare time rather than a true error.
An example was sold in Noble auction 95 in November 2010 as lot 138.
In 1994 the Royal Mint struck 1,000 gold proof two pound coins commemorating the 300th anniversary of the Bank of England, and silver and nickel-brass examples were also struck. A small number of the gold proofs were struck with a double sovereign obverse die rather than a two pound obverse die however, creating a rare mule. The standard coins have the obverse legend ELIZABETH II DEI GRATIA REGINA F D TWO POUNDS while the mule coins have the obverse legend ELIZABETH II DEI GRA REG F D.
It is unclear how many of the mules were struck - apparently some were recalled but some were also released and sold.