While Australian decimal mules are not uncommon, genuine production error mules as opposed to 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 it easy to see how such a mix-up could have occurred.
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 (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 all known examples were destroyed. Evidently a small number of mules went undetected 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 the rarest decimal varieties.
Although the Royal Australian Mint’s website records 1992 mob of roos one dollar coins as having been struck for circulation (https://www.ramint.gov.au/one-dollar) it is almost certainly in error. No 1992 one dollar coins were issued for circulation at all: a one dollar coin commemorating the 1992 Barcelona Olympic Games was issued in various mint and proof sets in 1992 and it was intended to be the only one dollar design for that year.
While the Royal Australian Mint’s website states that 0.008 million (8,000) mob of roos dollars dated 1992 were produced for circulation, this is a transcription of a likely erroneous entry in the 1992 Royal Australian Mint Annual Report. It is widely believed that the 8,000 one dollar coins in question were actually dated 1984 and 1985 and were retrospectively produced in 1992 for a set of five one dollar coins containing 1984, 1985, 1986, 1988 and 1992 one dollar coins. The complete lack of any reported 1992 mob of roos dollar from circulation helps to confirm that the website is in in error and that no such coin exists.
In recent years the existence of a proof 1992 mob of roos dollar has come to light. An example sold in Downies auction 310 as lot 2493 in February 2012 (https://issuu.com/downies/docs/auction_310_session_6) and was described as “One Dollar 1992 with Mob of ‘Roos reverse struck on a partially prepared proof blank, As Struck and extremely rare”. Curiously the lot was not illustrated but it sold for $1000 against its $750 estimate regardless. The circumstances of its production remain unclear but it seems probable that it was an unauthorised production by mint staff given that it was struck on a partially-prepared planchet.
The Indian Princely State of Sailana was comparatively late in its issue of its uniform-like coinage. India began its issue of uniform coinage in 1862 and the other Princely States that issued coinages in the same form all did so in the later part of the 19th century: Sailana however began its uniform-like coinage in the early 20th century (p389, Wiggins, British Commonwealth Coins, 1971).
Before Sailana's uniform-like coinage it issued copper paisas between 1871 and 1887 (p389, Wiggins, British Commonwealth Coins, 1971). It did not issue any further coins until its uniform-like coinage which consisted only of quarter annas in 1908 and 1912 (p389, Wiggins, British Commonwealth Coins, 1971).
The 1847 sixpence is one of the rarest British sixpences. No examples are recorded as being struck and there are no examples in the Royal Mint collection (p9, Coin Monthly, Jul 1973). Davies mentions 1848-dated sixpences with the last 8 over a 7 (p64, Davies, British Silver Coins Since 1816, 1982), as does Rayner (p160, Rayner, English Silver Coinage Since 1649, 1992) suggesting that 1847-dated dies were prepared.
A single well-worn and graffitied example was brought to light in 1973: the Royal Mint inspected it and found it to be of the correct finess and that the date did not appear to be modified but declined to confirm it as genuine as they had no records of any being struck or any examples to compare it against (p9, Coin Monthly, Jul 1973). The British Museum could also find nothing to suggest it was not genuine but also declined to confirm it as genuine (p160, Rayner, English Silver Coinage Since 1649, 1992). In 1982 Davies still listed in as unconfirmed (p64, Davies, British Silver Coins Since 1816, 1982).
The single known example sold in Dix Noonan Webb's auction on 29th September 2010 as Lot 1773.
The introduction of a new obverse on Australian circulating coinage in 1999 saw small changes to the obverse dies for a number of a denominations part-way through the 1999 mintage. The reasons have not been elicited but it is likely that the changes were to optimise production of coins with the new design.
The 1999 two dollar coin was produced with two different obverse dies - the first and less common variety being with a smaller Queen's head and legend further from the rim and the second, more common variety being with a larger Queen's head and legend closer to the rim. The relative size is the most obvious difference but it is hard to spot without an example to compare to.
Survey by author of 1999 $2 varieties from circulation
Although the small head design is less common it is not rare and may be more common in mint sets. Examples of the small head from circulation tend to be in poor condition too.