Not only counterfeit parts from bogus suppliers, but substandard parts from "approved" suppliers.
There's been a recurring problem - going back decades - with fasteners that look OK, dimensionally OK, static strength OK, but they don't meet the fatigue strength spec. It's affected all the engine mfrs. and tinknockers at one time or another. It results from cost-reduction shortcuts at the suppliers' shops.
FAA has cracked down - even the FBI was involved - and the problem has quieted down from its worst days.