Problem: you use the epub validation tool at Threepress Consulting, and consistently receive this error:

WARNING: [your epub file name].epub: item (iTunesMetadata.plist) exists in the zip file, but is not declared in the OPF file

Reason: you tested your epub by dropping it into iTunes. That’s all you have to do for this rogue file to be injected into the epub and make it invalid. You don’t even have to sync it to a device. You would expect iTunes to simply copy the file without manipulating the original’s contents, but such is not the case.

Solution: you can unzip the epub (on the Mac, I like the the ePub_UnZip_1.0 and ePub_Zip_1.0.3 Applescripts), delete the iTunesMetadata.plist file, and rezip it, or you can avoid the problem entirely by duplicating your epub file and dropping that into iTunes, leaving the original intact.