
Both times trying to upgrade to iOS 6.1.2 it has caused the iPhone to have to be restored to factory default. While this normally goes without saying I am putting this out there to drive it home for this update especially.
BACK UP YOUR APPS AND DATA!
Both my 3GS and 4 failed to update and had to be restored to factory default (3GS took 2 attempts to restore but could have been the cable I used) and I had to restore my data from there.
Once that is done however Jailbreaking it again is all fine and same procedure as always!
Get Evasion here
-Kouen