From EFF’s article
That's because your software [such as Adobe Photoshop] is secretly looking for security features such as EURion dots in the documents that you scan, and is hard-coded to refuse to let you make a copy if it finds them, even if your copy would have been for a lawful purpose.
One of the reasons why strict open source licenses are such a good idea: The software recognizing the EURion constellation and other security characteristics are provided as a proprietary binary blob and not included in Gimp and other OSS.
