Very doubtful that a Werke Nr would be applied in error. Apart from the known 'precision' of all things undertaken by Germany, especially formal records, still prevalent today, the number was the identity of the aircraft from the works order drawings, through the whole process of manufacture of each and every part, to final assembly and the remainder of it's 'life'.
It is highly unlikely that an error in applying the number on the fin, whether painted or by decal, would pass the final works inspection. Even if, at some time in 'the field', some overpainting, for example by mottle, or damage had occured, this would almost certainly have been rectified to show the number correctly. Afterall, any work needed on the airframe would be logged on the service records by the Werke Nr., and the only way of a 'mechanic' ensuring the correct aircraft was being worked on would be by seeing this number on the airframe, and on the data plate fixed to the port, front, main fuselage section, each being checked against the other.
Other markings on airframes might at some time be obliterated or partly obscured, but the W.Nr was, AFAIK, always kept visible.