New Zealand 1969 50c struck on British penny plancet
The Royal Mint was extremely busy in the late 1960s - striking enough predecimal coins to last until Decimal Day in 1971; striking new decimal coins that were slowly being phased in 1968; the various contracts that it had for overseas coinage and the gradual relocation of the mint to Llantrisant in Wales. Given all of what was going on it is not surprising that a number of error coins date to this era. One such coin is a 1969 New Zealand 50c piece struck on a British penny planchet.
The 1969 New Zealand 50c piece struck on a British penny planchet is a particularly obvious wrong planchet error given that the colour is incorrect: a New Zealand 50c piece should be silver-coloured cupronickel while British pennies are a brown bronze-like alloy. The planchet sizes are similar but different enough: the 50c piece should be 31.75mm across, 2.33mm thick and 13.61g while a penny should be 30.8mm across, 1.5mm thick and 9.1g - certainly of a similar enough diameter to be struck well enough, though the one known example shows some weakness in the details of the Endeavour (probably due to the lower thickness) and some fish-tailing on the legend (probably due to the smaller diameter).
Currently a single example is known (PCGS 15605990) which was last sold by Sterling & Currency for $750 in October 2024 (https://www.sterlingcurrency.com.au/new-zealand-1967-50-cent-error-struck-on-penny-pla).