This error occurs when a user tries to fulfill an eBook using a download link that was previously used with a different Adobe ID.
For example, let's say your user purchases the eBook titled 'The Art of War' from you and receives their download link for it. They use the link to download and open the eBook in Adobe Digital Editions with Adobe Id 'firstname.lastname@example.org'. A few weeks go by, and the customer also wants to download the same eBook onto their laptop. They use the same download link they used before, but this time with Adobe Id 'email@example.com'. That's when they'd receive this error. To correct this problem, they'd have to use 'firstname.lastname@example.org' as before.
If the user happened to fulfill the download of the book before authorizing ADE, it was then tied to an "anonymous" account and the title only exists in your machine library at that point. It would have been deleted when you reinstalled ADE or opened ADE on another device if that's the case.