In L'an 5 of their republican calender (1797 or so in normal time), the French revalued their copper decimal coinage. For the following 2-3 years, efforts were made to restrike the existing 1 and 2 decime coins to 5 centimes and 1 decime respectively. Some of this was done by simply striking the word UN over the 2 on the decimes. More interesting, though, were the efforts to restrike the new coins directly on the old. The results were bad, sometimes spectacularly so:


Above are photos of a 2 decime restrike, with a worn 1 decime to the left for comparison. The obverse shows traces of the old reverse wreath (like a collar around Liberty's neck, and above her head), plus part of the word "Decime". The reverse restrike leaves the original Liberty head clearly visible (it's upside down - note the nose and chin outlined - sorry for the poor photo quality), along with parts of the obverse wording (note the E of FRANCAISE).
Beyond these deficiencies, these coins were sometimes struck on the wrong planchets, which were sometimes made of bell-metal (kind of a greyish color compared to copper). The coin weights are highly variable - plus or minus 20% on the 20 gram weight is not abnormal.
I think of these defects whenever I see the high prices paid for VAMs, 1955 double dates, defective planchets etc. The Republique Francaise must be given credit for achieving perfection in this category...