"At its peak, Apple was seeing 60% of warranty repairs in China and Hong Kong as being fraudulent, literally costing Apple billions of dollars per year"

9to5:

Initially, Apple stopped allowing walk-in repairs and required reservation systems that supposedly ensured proof of ownership was provided. The system was beaten by hackers who exploited vulnerabilities in the web system who sniped all the time slots.

Apple then required candidate iPhone devices to run software diagnostics which would identify any fake parts inside, without requiring store staff to disassemble components and perform inspections. The thieves circumvented this by simply making the iPhones not turn on.

Some criminals were even more sophisticated.