I experienced a major problem and could not recover from a restore error on my iPhone 4 after attempting to upgrade to iOS 6.
I kept getting the dreaded "Verifying Software with Apple" stuck on with the progress bar on the iPhone never getting past one tenth complete.
My iPhone was connected to my Mac Pro Early 2008 using an Apple Cradle with an Apple 30-pin to USB connector to the rear of my Mac Pro's USB input.
This error sequence continued to repeat many times until I disconnected my iPhone from the rear USB input and reconnected to the front panel USB input on my Mac Pro.
Lo and behold my iPhone finally completed the upgrade to iOS 6 successfully!