Variety Vs Error Coin

Easy
If it was something to do with the die it's a variety.
Type 1: different die being made or used i.e. big head / small head 20c, or an incused, or a mule
Type 2: a die starts to fail i.e. die crack or Cud.
Usually a variety can be seen on numerous examples of coins, and are consistent.

An error is when it wasn't supposed to have happened in normal stamping production.
I.e. a misstrike, broad strike, out of collar, brokerage
There are usually very few examples of these, and you will rarely get 2 the same.

Then there are coins that I consider neither error nor variety:
Like laminated planchet peels, as these appear after a period of time due to strain on the planchet bringing them to the surface. They are the product of a flawed planchet with impurities.

CCF Numismatic Glossary

error
A numismatic item that unintentionally varies from the norm. Ordinarily, overdates are not errors since they were done intentionally while other die-cutting "mistakes" are considered errors. Double dies, planchet clips, off-metal strikings, etc. also are errors.

variety
A coin of the same date and basic design as another but with slight differences. PCGS recognizes all major varieties while there are thousands of minor varieties, most of which have significance only to specialists of the particular series. After hubbed dies, introduced in the 1840s, varieties are mainly variations in date and mintmark size and placement.

I wouldnt classify a die crack or Cud as a variety, rather an error as it was not tooled intentionally into the die.

with ozcoin I would classify a mule as an error

Thanks to all for replies

In particular, how would you class the 2 1981 20c coins?
Were the dot Cud + die chip part of the dies used?

with Australian Coin. An Error can turn into a Variety, but a variety can't turn into an error. There are many examples of Errors made on dies but these turn into varieties because thousands are made. A Cud is a variety, a die crack is a variety,the 2000 dollar mule is a variety,the 1931 dropped 1 penny is a variety,the 1962 double nose penny is a variety.Date doubling is a variety.The 1981 20c are varieties. Although they are technically errors they magically turn into varieties as thousands are produced.
Planchet flaws are errors as they are usually unique, misstrikes are errors as they are usually unique.

These are normally referred to as ERROR varieties and not as varieties in their own right
An error is an error regardless of the amount minted,Just because thousands were produced they are still error coins.
A variety is a deliberate change in the design by the mint

devious buggers us numismatists