1988 Coat of Arms Mule 50 Cent
A number of interesting mules have been produced at the Royal Australian Mint over the years, mostly in the 1970s. One mule produced in the 1980s saw the combination of a 1988 50 cent obverse die with a regular Coat of Arms 50 cent reverse. All circulating and mint set 1988 50 cent pieces were meant to have been produced with a commemorative bicentenary reverse. There is reported to be a single example in uncirculated condition which first came to light in 2006 (p135, McDonald, Pocket Guide to Australian Coins and Banknotes, 2014) but there may be others. The circumstances of its production are unknown but given that a single uncirculated example is known, it was probably a deliberate production on the part of mint staff not following procedure.
An example was sold as lot 487 in IAG sale 64 in August 2006 for $5,400 plus commission. The November 2019 Australasian Coin and Banknote Magazine reported that Downies sold an example to a private buyer for an undisclosed five figure sum.
Apparently the muling was not limited to circulating coins. In more recent times (unclear when exactly), two silver proof mules have also appeared: https://www.pcgs.com/cert/36144387 and https://www.pcgs.com/cert/81319131. There is little information on these coins but they are likely to have been part of Bicentennial Masterpieces in Silver sets.