Following reports of bricked iPhones, Apple has confirmed that "error 53" is real -- but it's not what is causing the bricking.
The Guardian reported last week that after installing the latest update for iOS9, "thousands of iPhone 6 users claim they have been left holding almost worthless phones". This is due to the iOS9 permanently disabling the handset if is detected that repairs have been carried out with non-standard components on the handset.
Apple has said in a statement to Gizmodo:
We take customer security very seriously and Error 53 is the result of security checks designed to protect our customers. iOS checks that the Touch ID sensor in your iPhone or iPad correctly matches your device's other components. If iOS finds a mismatch, the check fails and Touch ID, including for Apple Pay use, is disabled. This security measure is necessary to protect your device and prevent a fraudulent Touch ID sensor from being used. If a customer encounters Error 53, we encourage them to contact Apple Support.
So the bottom line is, if you have an iPhone with touch ID fingerprint recognition, and have it repaired at an unauthorised repairer using non-standard techniques or components, iOS9 detects this and locks everything down as a security measure. This way no one can access your device and potentially private data, including the cards you have linked to Apple Pay.
Apple said in a statement "When an iPhone is serviced by an unauthorised repair provider, faulty screens or other invalid components that affect the touch ID sensor could cause the check to fail if the pairing cannot be validated. With a subsequent update or restore, additional security checks result in an 'error 53' being displayed."
Kyle Wiens from iFixit, "the free repair guide for everything, written by everyone", believes there is something more sinister behind the feature, telling The Guardian that "all along, Apple’s view is that it does not want third parties carrying out repairs to its products, and this looks like an obvious extension of that."
Of course, Apple suggests only taking your iPhone to Apple for repairs to prevent any issues occurring, but the better repair places are aware of this feature. To ensure your phone stays in good working order, they will manually move the touch ID sensor from the broken glass face plate to the new face plate when repairing a cracked screen.
If your iPhone has been affected from non-standard components being detected by the latest iOS9 update, your best bet is to get in contact with Apple.