Here's an interesting observation, in case it makes anyone feel better:
I manage a fleet of about 75 iPhones for our company and I decided to update a phone I had sitting on my desk that is about to be assigned to a user. I used iTunes to do the update (instead of OTA) and I ran into the same issue- it froze at about 1/4 of the progress bar filled.
Now what was interesting was that iTunes showed the device as connected, responding, ready for backup/sync/etc. Usually, from beginning to end during a firmware update via iTunes, there are various status messages and progress bars that show in iTunes. These don't go away until the final reboot that brings you back to the home screen.
So, if I had to guess, in each case where the simple reset fixed the issue there was no "damage" done. It looks like the device merely got stuck displaying the progress bar and was in fact in a safely updated state and awaiting a reboot.
No gaurantees here- just my 2 cents and observations from an IT professional.