In 2015 the Reserve Bank of Australia had $5 and $10 notes printed with the signatures of Reverse Bank Governor Glenn Stevens and Secretary to Treasury John Fraser (https://banknotes.rba.gov.au/resources/for-collectors/serial-numbers/). In the lead-up to the release of the Next Generation Banknote $5 note in 2016, very few of the 2015-dated $5 saw any use.
According to the Reserve Bank, 9,715,400 2015 $5 notes were printed (https://banknotes.rba.gov.au/resources/for-collectors/serial-numbers/). The Reserve Bank have advised that some were released into circulation and 84,000 were released in sets alongside the new Next Generation Banknote $5 note in 2016.
No doubt the Reserve Bank wanted to avoid releasing banknotes with an old design alongside the new design banknotes which is why few, if any, were released into circulation. Most of the 2015-dated $5 were probably destroyed.
In the 1960s the Royal Mint was striking coins for many overseas countries; not just Great Britain and its Crown Dependencies. In 1966 many British pennies were struck in the lead-up to decimalisation, and a comparatively small number of Jersey 1/12th of a shilling were also struck. There was apparently a mix-up during production of the coins as a small number of 1966 British pennies were struck with the obverse for a Jersey 1/12th of a shilling.
Just two examples of mule are known: one was sold as lot 124 in the Colin Cooke auction of The James Workman Superior Collection of Pennies Part 2 in November 2010 but no provenance was given. The other example has not been sighted in a sale in recent times.
Through the 1960s, the Royal Mint was busy striking coins not only for the United Kingdom, but also for a number of other members of the Commonwealth: the decimalisation and independence occurring in many of these countries created a busy time for the Royal Mint. Although New Zealand had not yet converted to decimal currency and was already independent, their bronze and silver coins were part of the Royal Mint's workload in 1965: it is in this environment that a 1965 New Zealand halfpenny was accidentally struck with a British halfpenny obverse.
Only one 1965 New Zealand halfpenny mule is known and it shows signs of a small amount of circulation - it is possible that more may exist but more than likely the mix-up was discovered and almost all examples were caught. The provenance of the one example is unknown but it was last sold as lot 1602 in Downies auction 313 in February 2013. It had previously been listed in British coin catalogues (p30, Perkins, Collectors' Coins GB, 2011).
Australian wrong-planchet errors are rare with only a handful of different instances known. One such instance is a 2009 mob of roos $1 struck on a bi-metal planchet, which just a single example known.
The bi-metal planchet on which the 2009 mob of roos $1 was struck has a gold-coloured outer rim with a silver-coloured core. Tellingly, the rim has edge lettering BCV 1 - the same edge lettering used on the coins of Venezuela (BCV stands for Banco Central de Venezuela i.e. The Central Bank of Venezuela). In this case the bi-metal planchet was intended for a Venezuelan 1 Bolivar coin but was accidentally shipped to Australia along with normal $1 planchets for striking.
In July 2007 a 2001-dated 20c piece struck on a bi-metal planchet was sold in Downies auction 295. It was reportedly found in an Armaguard roll. The piece was struck on a planchet with a gold-coloured outer rim with a silver-coloured core and it weighed 10.66g: not only was the planchet the wrong colour, it was a little under the designated weight of 11.31g (p124, McDonald, 2014 Pocket Guide to Australian Coins and Banknotes, 2013).
A second example of this error was shown on Facebook at some point in 2017 (p10, Andrews, Errors and Varieties in Australasian Coin and Banknote Magazine, September 2017). It was sold in October 2017 at Downies auction 325 as lot 1785.
The Royal Australian Mint source planchets from Poongsan in South Korea. The company produces planchets for many countries and their standards are high: contamination of even a single wrong planchet is rare, let alone multiple. Nevertheless, at least two bi-metal planchets were shipped to Australia along with millions of other 20c planchets at some point in 2000 or 2001.