2006 silver proof 2c mule with IRB portrait
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.
Reportedly just two coins are known (Eigner, https://www.drakesterling.com/news/post/modern-coin-rarities-of-australia) though there may well be others.
While fabricated errors are not uncommon from the Royal Australian Mint, these mules seem genuine as they were found in sets rather than first appearing at auction.