When reviewing eproofs and PDFs, you will often see hairlines. These thin, light colored lines are a result of a transparency flattener used by PDF creators and will sometimes appear around text boxes and masked images. Fortunately these hairlines are only visible in your PDF and will not print when the book is produced. These lines may appear on both cover and interior files.
Please note that while the above examples are most common, hairlines may also present themselves in additional ways. If you have any concerns, it’s advisable to review your native working files to ensure the lines aren’t present. Physical proofs are also available for review.