[http://Here is ]what Apple suggest " Custom firmware restore fail (PwnageTool since 4.1). You have to put your device into the DFU mode and not into recovery mode. Device won't boot correctly an stuck at the Apple logo after the error. Press and hold power and home button until it reboots.
Error when using Sauriks server for a restore with iOS 5.x in the recovery mode at least on a A5 device. " If you need further help, check [http://www.iphonestuffs4u.com/iphone-error-21/|on here.]