If you get a I/O Error - Invalid Zip File or Inconsistant Zip Archive error when validating an ePub file using - http://validator.idpf.org/Download StuffIT Expander from the App Store, the only other tool you will need is terminal.1. Rename the broken ePub with a .zip extension.2. Using StuffIT Expander, unzip broken archive.3.  Open terminal and navigate to the correct directory where you expanded the archive.4.  Run the following commands in terminal:

1. zip -X fixed.epub mimetype
2. zip -rg fixed.epub META-INF -x \*.DS_Store
3. zip -rg fixed.epub OPS -x \*.DS_Store

Note that the OPS folder is sometimes OEBPS, this must match the original folder structure.