The problem with spurious Mint emissions is that a deliberate anomalous product commands a much higher price than a random blank picked up and thrown in the bucket for use. i.e. if you could prove it was a striking specifically tied to a certain mint action, such as trying out a regular mix on a different thickness blank, or a test for a new currency contract from a foreign country, you will do better than the unintended oddball. Wrong flans occur regularly, which is not surprising given the billions of blanks cut every year. Clearly even a hundred wrong'uns would be extremely rare, but in the context of mint operations would only amount to a rounding error if checked for weight.
So to consider the above point, it is definitely not common, certainly excessively rare (in the context of a currency issue) and maybe even unique. The problem lies in documentation, because errors are by definition, not struck for the records.