1974 Double Headed Australian 20c
In general, major mint errors in Australian decimal coins are fairly uncommon, but double-headed or double-tailed coins are especially rare: the presses apparently make the production of such coins difficult, and possibly impossible in more recent years. Nevertheless, the Royal Australian Mint has been responsible for a number of double-headed and double-tailed coins over the years.
Presumably in 1974, a double-headed 1974 20c piece was struck, with the heads sides being at 180 degrees to each other. This appears to be the only such known double-headed 1974 20c piece, and it is likely that the sides being at 180 degrees to each other is because the press did not allow the dies to be inserted at the same orientation. A number of other such error coins are known from the 1970s and it is likely the result of mint workers experimenting in their spare time rather than a true error.
An example was sold in Noble auction 95 in November 2010 as lot 138.