With the iPhone 6 (and later) which have been in circulation for a little over a year now there are certain caveats that must be observed when repairing them. The two that are related to Error 53 are:
So, whilst we know this and when we repair iPhones we ensure that we keep the same touchID sensor and put the right screws in the right places, this isn't always the case in the third party repair marketplace. The screws in the wrong holes are unforgivable but the TouchID sensor is a little more complex because the issue only appears when you try and upgrade the iPhone to IOS9, so as long as your on IOS8 or earlier you're going to be fine, try and upgrade and your phone is dead.
The real issue here however, is not that the TouchID sensor was discarded/replaced in the past by a third party repairer, its that Apple have decided in their ultimate wisdom to implement a 'check' in IOS9 which will retrospectively render customer's iPhones useless when they upgrade, and without any warning at all. We and the whole industry assumed at first that this was an oversight or error, but Apple have made no efforts to resolve it rendering customers phones worthless in the thousands. For Apple its a win because the customers now have to purchase a new iPhone and hope that they have a backup of the dead one, but that's not always as simple because the newly purchased iPhone will be IOS9, and the backup will most likely be IOS8 which of course will not restore to an IOS9 phone - thanks again Apple. There are third party Applications such as DiskAid, etc which can transfer the majority over but its hard work and a world away from the 'everything is simple and intuitive' that Apple likes to imply.
The future is uncertain, and it all depends on how many users are ultimately effected and whether any regulator steps in to enforce some sort of resolution, we'll just have to see.