Die Adjustment Strikes
Feeder Finger Strikes
Gold Die Trials
Pattern Die Trials
Uniface Die Trials
Euro 50 Cent/20 Cent Mule
An example would be a New Zealand/Bahama mule where thousands were struck and they are worth $50 each. The obverse of a Bahama 5 Cent piece was muled with the reverse of a New Zealand 2 Cent piece.
On the other end of the spectrum are the extremely rare mules including a few from the United States. An example would be a mule with a Sacagawea Dollar reverse and a State Quarter obverse.
The photo shown above is a rare example of a Euro 50 Cent reverse that was muled with a Euro 20 Cent reverse. This rare piece is also a "two-tailed" coin because both dies were reverse designs and both are denominations of Euro coinage.