1978 Fiji 50c mule
A number of interesting mules were struck at the Royal Australian Mint in the 1970s - it is widely believed that staff at the time were engaged in illicit experiments, though the availability of their work would suggest that they "got away with it". One such piece is the 1978 Australian 50c struck with a Fijian 50c reverse.
The circumstances in which the 1978 mule were struck are unknown, but the Royal Australian Mint struck both Australian and Fijian 50c pieces in 1978, so it was without a doubt struck at the Royal Australian Mint. The number struck is unknown but it is assumed that at most a handful were struck. In any case, it it not the only 1978 Australian/Fijian mule, with both 5c and 20c pieces existing with Australian obverses and Fijian reverses.
An example was sold in International Auction Galleries' September 2024 sale for $14,400.