What does the iTunes message "The phone could not be updated. An unknown error occured (11)." mean? If you are on Windows, it could mean that your hard drive is full. For me the download worked fine, it was saved to D:, but during the extraction process C: filled up. iTunes spits out error (11) and cleans up the temporary files it was extracting.

The "More Info" button sent me to this page, http://support.apple.com/kb/TS3694, which at the time of this writing did not have error 11 listed.

Solution: You need about 700MB free on C: for the extraction/verification process to work.