1988 Cook Islands/Australia 5 Cent Mules
In 1987 and 1988 the Royal Australian Mint struck coins for the Cook Islands as well as for Australia. During this time a mule of a 1988 Cook Islands 5c obverse and an Australian 5c reverse was struck. How or why it was struck are unknown, as are the details of its discovery. The 1988 Cook Islands 5c may also have been a mint set only coin.
It is likely that there is only one example known, though possibly a small number of others remain dormant in the market. There was a sale in Status International's June 2005 auction with a $750-$1,000 estimate (Hartshorn, http://members.optusnet.com.au/~ihartshorn/feature/f2.htm), another sale in Roxbury's auction 96 in May 2018 as lot 659 and another sale in Status International's June 2023 auction 381 as lot 11579.
Interestingly in Status International's June 2023 auction 381, a 1998 Australia 5c struck with a Cook Islands reverse was sold as lot 11760 and a double reverse Australia/Cook Islands 5c mule was sold as lot 11761. These most recent sales suggest that 1998 was the likely production of the three coins and that they were deliberate fabrications rather accidents.