2000 Dollar Mule
While Australian decimal mules are not unheard of, genuine production error mules as opposed to creative mint workers intentionally mixing and matching dies almost never occur. The first and only known occurrence to date at the Royal Australian Mint was in 2000 when a small number of mob of roos one dollar coins were accidentally produced with a 10c obverse die. Given the small difference in size between the one dollar and 10c obverse dies - a mere 1.6mm - it easy to see how such a mix-up could have occurred. Genuine 2000 dollar mules are quite easy to spot by their double/stepped obverse rim. The rim is not always centred on the mules though, and well-centred mules are more desirable.
The 2000 dollar mule was first publicised in the February 2002 Australasian Coin and Banknote Magazine where a reader named Dave Potts on the New South Wales Central Coast reported an unusual 2000 one dollar coin which he and Alan McInnes concluded was struck with a 10c obverse die. In the September 2004 Australasian Coin and Banknote Magazine Ian McConnelly offered an update on the 2000 dollar mule and reported at least 20 known examples with the locations implying mules being found most commonly in New South Wales, Victoria and Western Australia. Apparently a majority of the mules were released into circulation in Perth with a large amount being found there (Harris, https://www.australian-coins.com/error-coins/2000-1-10-cent-mule/).
The Royal Australian Mint has been quiet on the subject but reportedly an employee anonymously divulged that the mistake was contained to a single incorrect obverse die and when the mistake was discovered during production, there was an effort to destroy all of the mules. Evidently a small number of mules went unfound and were subsequently released into circulation. It has been reported that all known mules have identical reeding alignment which would confirm that the mules came from a single die run.
The number of mules that entered circulation is unknown and probably never will be but collector Bruce Mansfield advises that in the 31,546 2000-dated one dollar coins he searched, he found 41 mules. From this sample it can be assumed that 0.13% of 2000-dated one dollar coins were mules, and given the total mintage of 2000-dated one dollar coins is 7,592,000 it can be deduced that approximately 9,800 mules entered circulation. Regardless, the 2000 dollar mule is one of Australia's rarest decimal coins.