Don't use Archive Utility to unpack ZIP files on Mac. It follows standard Mac philosophy to make users feel better by not reporting any errors to them. If you have a corrupted ZIP file, and you try to unpack it using Archive Utility, you won't get any error message, it will just silently ignore any files that were corrupted and couldn't be unpacked. (Just the same, if you try to read files from a broken CD/DVD, Mac OS X won't tell you it couldn't read them, it will just silently put zeroes in all places of the files that couldn't be read).
I prefer to use Unarchiver (
http://wakaba.c3.cx/s/apps/unarchiver.html ). It's free and open source.