Error coins are coins that have been miss-produced at the mint.

Most should have been spotted at the mint before being released into circulation,
therefore, all tend to be relatively rare.  

Some error coins depending on the type of error are extremely rare. Gold ones tend
to be amongst the rarest because they are the coins that are more closely inspected
by the mints.  

Error coin collecting is a more specialised coin collecting subject and although
initially error coins were scorned at by collectors, the popularity for collecting them
has increased over the recent years.

Below are several types of major error coins.
ERROR COINS
Broadstruck Error
Brockage Error
Blank Planchet Error
Clipped Planchet Error
Die Cap Error
Doublestruck Error
Off Centre Error
Thick & Thin Planchet Error
Wrong Planchet Error
Multiple Error
GEORGE IV
GEORGE III
GEORGE II
GEORGE I
ANNE
WILLIAM III
WILLIAM & MARY
VICTORIA
EDWARD VII
ERROR COINS
GEORGE V
GEORGE VI
WILLIAM IV
EDWARD VIII
GRADING COINS
PATTERN COINS
CROMWELL
JAMES II
CHARLES II