Restore Errors/Below 100

Error 1

 * You are attempting to downgrade an iPhone from iOS 5 to iOS 4.x. To resolve, use either custom firmware created by PwnageTool/sn0wbreeze or use notcom's FixRecovery utility available on his site.
 * On an iPhone 4 downgrading from iOS 5, use the option in TinyUmbrella to allow a baseband "upgrade" to bypass the error and go back to basband 4.10.1.
 * Unable to downgrade. Try changing the USB port (the back one of chassis is better) and restart computer.
 * The installed version of iTunes may also be too old. Update iTunes.
 * You are trying to downgrade an iPhone 4 to an earlier version of iOS 5, using redsn0w. To resolve, stitch your blobs to a "No_BB" ipsw of the firmware you are trying to downgrade to.

Error 2

 * ASR does not exist on ramdisk or is corrupt/not signed.
 * sn0wbreeze 1.6 Custom Firmware has a ASR patch problem. Use sn0wbreeze 1.7+ or PwnageTool. Device isn't bootable.

Error 6
Not enter the downgrading mode, change USB port (the back one of chassis is better) and restart computer.

Error 9

 * Due to asr being patched, the SHA signature is automatically changed and after being resigned the kernel will refuse to use it. Therefore proper kernel patches are required. If necessary kernel patches are not applied, it will fail to load asr and error 9 would occur during restore.
 * Rebooting your Mac or PC may resolve this issue.

Error 10
LLB is missing from the IPSW. The device cannot be booted up. The trick to skip the baseband update no longer works.

Error 11

 * Removed BB FW file in the firmware folder of an unzipped IPSW. Can also be used to prevent a baseband update. The device is bootable, if you set the auto-boot to true or use TinyUmbrella and use "Kick Device out of Recovery".
 * You are trying to update to a custom firmware from PwnageTool 4.1+. You always have to go into DFU mode and restore from there.
 * Occurs when attempting to restore a signed custom firmware on the iPhone 4 using iTunes 11.1 or newer. Downgrade to an earlier iTunes to fix.

Error 13

 * Occurs when you want to install an iOS 4.1 beta firmware with iTunes for Windows (actively blocked by Apple; beta users are developers and therefore must have a Mac).
 * USB problem. Check the USB connection and try other direct ports or maybe the USB cable is an older one. Device isn't bootable.
 * The installed version of iTunes may also be too old. For firmwares 4.0 and higher you need iTunes 9.2 or higher. Update iTunes.

Error 14

 * Custom firmware update fail (PwnageTool until 4.0.1). You have to restore the device with a custom firmware. Update to a custom firmware isn't working. Device isn't bootable. 2
 * USB Problem. Check the USB connection and try other direct ports or maybe the USB cable is an older one. Device isn't bootable.
 * You're trying to 'update' to a beta firmware instead of performing a restore. Device is still bootable if you kick it out of recovery mode.
 * Rebooting your Mac or PC may resolve this issue.
 * Root file-system partition size was set too small in the ramdisk's options plist.

Error 17

 * Device failed to display the applelogo img3 which results in this error.
 * One or more of the nand0/nor0 flash images were missing from the IPSW.

Error 18
This occurs when the media library on the device is corrupt and cannot be modified or updated.

Updating to the latest version of iTunes and then restoring should resolve this issue

Error 20
Reportedly happens during failed downgrade attempts (iPhone OS 2.0 to 1.1.1, iOS 5 beta to 4.3.3).

Some have seen success afterwards by doing the downgrade from DFU mode instead of the normal Recovery mode.

Error 21

 * 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.
 * This error can also happen when you try to restore an iPod with hardware dfu. Use iREB r5 to solve this.
 * You are trying to update to a beta firmware without UDID activation.

Error 23
Cannot restore or update due to hardware problems.
 * In software you have to set default web browser.
 * In hardware, it's a communication problem. Check all connections, also try to change battery.
 * Faulty baseband flash or processor
 * Happens frequently after water damage. Remove and clean motherboard.

Error 26

 * Values or variables in the options property list within the ramdisk are incorrect.
 * False version of the NOR flash firmware. Use the correct sn0wbreeze version.

Error 27

 * Trying to flash iOS 5 from an iPhone 3GS on a 3G Phone. Modify the restore.plist so it would flash and also exchanged RestoreRamDisk and kernelcache.
 * Failed attempt of upgrading an iPod touch 5G from an older iOS to iOS 9.0 beta. Can be resolved by restoring to the latest public version and trying the upgrade again.

Error 28

 * Hardware error. Return your device to Apple if possible. The problem is a bad dock connector on the iPhone. Must change the connector.
 * If you have tried almost everything, reset the logic board (leave uncharged for five days or remove battery for half an hour), then pwn and restore. If this doesn't work, the memory of your device is damaged.
 * Hardware is incompatible with iOS Version. Typically seen with iPhone 4 GSM models trying to be restore to iOS 4.0. Can also be seen when trying to restore any geekgrade iOS 4 IPSW on an 8GB iPhone 4 (iPhone3,1).
 * Device has counterfeit display that is incompatible with a certain iOS versions (Typically iOS 5.X)

Error 29
See Apple's discussion thread number 2329795 about this problem. You might need to change the battery.

Error 34
Hard disk is run out of space when trying to download. Clear space and then continue downloading.

Error 37

 * iPod touch 2G LLB patched with the 0x24000 Segment Overflow was used on an iPhone 3GS custom firmware. Known on damaged bundles from unofficial PwnageTool distributions or bundles.
 * sn0wbreeze 2.0.1 bug, which was fixed in 2.0.2.
 * Recent sn0wbreeze can cause this issue. Deselect all custom boot logos, recreate IPSW. If received on an iPhone 3GS, select "new boot-rom" and deselect all custom boot logos, then recreate the IPSW.
 * Attempting restore with an IPSW that was stiched with blobs not for your device (Usually a good thing if restoreing with a geekgrade beta IPSW or with your own creation). To boot, simply open redsn0w. Do fix recovery and tether boot after. Do not lock or device's LCD will fail.

Error 40

 * Hacktivation bug in sn0wbreeze 2.0.1, which was fixed in sn0wbreeze 2.0.2.
 * NAND chip is dead (confirm with ASR output from restore log).

Error 53

 * Happens on devices with damaged Touch ID, cause is most likely water damage. Bring the device to Apple if possible.
 * Can be an error with iTunes. In this case, change your DNS to 8.8.8.8 and try again.

Error 56

 * Most likely a dead/glitched Touch ID sensor. If seen on iPad's, try restoring the latest firmware in DFU mode. If error still occurs, bring your device to Apple. On iPhones, take out the SIM card and try restoring in DFU mode. If still occurring, bring your device to apple.