- Custom firmware can't be restored on a device with DFU Mode. The software of the iPhone isn't patched. Using iREB, sn0wbreeze, redsn0w, or PwnageTool, you need to enter a pwned DFU Mode. During that process the software gets patched and the custom firmware should run without any problems.
- APTicket problems if you try to restore to iOS 5.x.
- Restart computer; change computer; reinstall system.
- If this error occured during an Apple TV restore, then unplug the Apple TV from the power cable, after you entered the recovery or DFU mode, and then try it again.
Device was not pwned (signature checks were not blocked). Custom firmware can't be restored. Can also happen when there's a problem with the IPSW.
iTunes tried to enter the Restore Mode, but the device turned back to the Recovery Mode. This can happen when updating an iPad 2 jailbroken with Saffron gets updated to iOS5. In this case simply retry, as it is in Recovery Mode then anyway.
Missing system files or corrupt data in iTunes installation. Try to uninstall iTunes, restart computer, install again. This error also occurs for iTunes in Windows 8 for reasons currently unknown.
iTunes is too old and couldn't read the iPhone in Recovery or DFU Mode. Please update iTunes and try it again.
iTunes expected the device to be in a different state (Example: booted into iOS rather than jumping to iBEC).