Among the many different mules struck at the Royal Australian Mint in the 1970s is the 1978 Australian 20c struck with a Fijian 20c reverse. There is no question that these coins were struck at the Royal Australian Mint, as it was responsible for striking both Australian and Fijian 20c pieces in 1978, but beyond that, details are scarce.
Wrong planchet errors are fairly uncommon in modern times as both mints and security companies have equipment that should identify coins which do not meet specifications. In any case, a small number of Australian $2 coins were struck with 10 euro cent planchets, which is especially impressive given that the coins should weight 6.6g but the planchets weight just 4.1g. None of the known coins are struck well enough to show a year, but a number of pieces have reportedly been discovered since 2012 (Eigner, https://www.drakesterling.com/australia-two-dollar-struck-on-foreign-planchet-pcgs-au55-47101599).
A number of interesting mules were struck at the Royal Australian Mint in the 1970s - it is widely believed that staff at the time were engaged in illicit experiments, though the availability of their work would suggest that they "got away with it". One such piece is the 1978 Australian 50c struck with a Fijian 50c reverse.
The circumstances in which the 1978 mule were struck are unknown, but the Royal Australian Mint struck both Australian and Fijian 50c pieces in 1978, so it was without a doubt struck at the Royal Australian Mint. The number struck is unknown but it is assumed that at most a handful were struck. In any case, it it not the only 1978 Australian/Fijian mule, with both 5c and 20c pieces existing with Australian obverses and Fijian reverses.
An example was sold in International Auction Galleries' September 2024 sale for $14,400.
As with many other denominations, the sixpence's obverse and reverse designs were updated in 1887. The sixpence's reverse design ultimately received two major updates as the first new reverse (a shield) was similar enough to the half sovereign's design that, coupled with the similar sizes of the sixpence and half sovereign, there were attempts to gold plate the new sixpences and pass them off as half sovereigns (Gouby, http://michael-coins.co.uk/sx1887%20JH%20var.htm). As a result, the shield obverse was short-lived.
Two different obverse and two different reverses were used for the 1887 shield sixpences: on Davies obverse 1 the designer's initials JEB are on the truncation of the portrait and on Davies obverse 2 the designer's initials JEB are below the truncation of the portrait and on Davies reverse A the tail of the Q in QUI is missing and the loop above the buckle between the date is incomplete while on reverse B (not recorded by Davies) the tail of the Q in QUI is visible and the loop above the buckle between the date is whole.
Not all obverse and reverses were paired with each other: 1+A coins (Davies 1150) exist, Davies 2+A coins (Davies 1151) exist and 2+B coins exist. 1+A coins are quite scarce, 2+A coins are common, 2+B coins appear to be mostly or entirely proofs and 1+B coins are not known but may exist.
In 2003 the Royal Australian Mint struck a commemorative $1 coin to commemorate 100 years since women's suffrage was achieved in Australia. Two slightly different reverse designs were used to strike these coins: the reason for the change is not known but it is assumed that the first design was prone to die deterioration or cracking and so an improved design was used to strike the balance of the mintage.
The differences between the designs are subtle: one on design the letters CEN touch, the RA run into each other and the gap between the rim and the women is small (the so-called large reverse) and on the other design the letters CEN are spaced out, the RA touch but do not run into each other and the gap between the rim and the women is large (the so-called small reverse).
Neither variety is rare and the split is approximately 70% large reverse and 30% small reverse.