1988 Cook Islands/Australia 5 Cent Mule
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) and another sale in Roxbury's auction 96 in May 2018 as lot 659.