If you have downloaded an ipsw (iPhone-iTouch System Software) from a direct apple link and not from iTunes itself, do not unzip it and zip it back. It has the .ipsw extension, but in fact, it is a zip file
The problem is that the zipped back ipsw might be different (MD5,SHA1 or whatever) .
Then when you will manually restore/update in iTunes, it will fail.
I used several zipped back ipsw files and got failure every times and after used an original ipsw and the retore/update worked.
Mac users using Safari should disable the "Open Safe files after downloading" feature in Preferences>General as it will unzip this file automatically, so you will think you'll have to zip it back again.
All users: make sure the ipsw file you download is from Apple. If you download it from a website/torrent/ftp... you cannot guess if the guy who put it there did the unzip/zip manipulation.
I think many users, without knowing it, have faced restore/update issues because of this.