Mule Coin Errors
What is a mule error?
A mule occurs when a coin is struck using two dies that were never intended to be paired — typically a current obverse with an out-of-rotation reverse, or a reverse from one denomination on a planchet for another. Mules are nearly always Royal Mint errors rather than counterfeits, and they are some of the most valuable modern UK error coins.
How to spot one
- Compare both sides against the official Royal Mint specification for that year.
- Check the date — mules often pair this year's obverse with an old reverse, or vice versa.
- Look for missing or extra inscriptions (e.g. a missing date that should appear on the obverse).
- Verify the diameter and weight match the denomination on the reverse.
Authentication
Mules are almost always genuine Royal Mint errors. Counterfeits do exist of valuable mules (especially the 2008 undated 20p) — check weight (5.00g) and diameter (21.4mm). For coins worth over £200, professional grading at PCGS, NGC or CGS UK is recommended.
Famous UK examples
New 20p reverse paired with old obverse missing the date — the most famous modern UK mule. Read the full guide →
Used the pre-1982 "NEW PENCE" reverse on Royal Mint sets, after the legend changed to "TWO PENCE".
Some examples paired old "STANDING ON THE SHOULDERS OF GIANTS" edge inscription with the Daniel Defoe reverse.
Key-date UK coins worth examining
Errors on key-date coins compound rarity — the host coin is already scarce, and the error multiplies the value. Browse the rarest UK coins in our catalogue: