1966 Wavy Baseline 20 cent
Circulating Australian 20c pieces were struck in both London and Canberra in 1966 and neither coin has any varieties. A small number of London-struck coins show an interesting error however - a single reverse die has an upward bump in the baseline of the 2, where it meets the downstroke, and this bump is matched by an inward bump in the wave directly above the baseline of the 2 - a wavy baseline. The cause is unknown but likely the die was bumped before use causing a small shift in the design.
While the error is the product of a single die, it seems likely that the die was either removed from service reasonably quickly, or did not last: the wavy baseline 20c is quite scarce.
The error has been known since at least the 1980s, though its pricing in early catalogues betrays its rarity: high-grade examples can sell for thousands of dollars, not the single digit dollars reported in the mid-1980s.