Double die error coins

When a (penny) working die was manufactured, it is typically struck three times by the puncheon. (Halfpenny dies were struck twice.) Between pressings the die is annealed and machined. When the die is put into the hobbing press for the next strike it has to be carefully aligned so that the punch strikes the die in exactly the same place. If the alignment is imperfect then the die will show displaced imprints of the earlier strikes. This is a well-known phenomenon and a die exhibiting such a feature is said to be a "doubled die".
A good illustration of this is this 1943 halfpenny specimen shown here in fairly high resolution:

Double die error penny

Obverse of a 1943 halfpenny showing a doubled legend produced by imperfect alignment of die and punch on the second pressing.

The double-nose Queen

The one variety recorded amongst the Perth Mint pennies of the sixties is the extraordinary "double-nosed queen" variety where the obverse shows multiple anomalies, the most obvious of which is a second nose on the face of Queen Elizabeth.

There are four anomalies clearly visible in this picture.
1. The nose is doubled.
2. The ribbon is doubled.
3. There is doubling of the legend on the right.
4. The upper chest is doubled.

Doubling of the legend on the lower left. This is not obvious from the main picture but shows clearly in the high-resolution image

Other Australian coins errors

Mint error types

The "clipped planchet" error

The 1916 mule halfpenny