blacklisted serial numbers

On very rare occasions, some camera models store an invalid serial number in the photo's exif data. For example, all Nikon D50s write "D50" as the serial number. Since they all store the same thing, it is of no use when uniquely identifying cameras.

Another cause is that some software overwrites the serial number when you edit the photo. It could be something as simple as rotating the photo and saving it. Where possible, please use an image file that has not been modified.