I imagine you've heard of the fake parts the airlines were not supposed to be buying and installing.

American Airlines, Southwest Airlines, and United Airlines

Earlier this month, news broke that British aerospace parts supplier AOG Technics had forged certification documents for dozens of parts used in the CFM56 turbofan. 

This is the world's most widely used jet engine, powering workhorses like the Airbus A320 and Boeing 737. 

The origin of the fake parts isn't yet known

https://www.thedrive.com/news/fake-jet-engine-parts-ground-american-southwest-united-planes-as-scandal-grows

I wonder if they will have the fake parts tested to see how far off-spec they are.

 If they’re way off, this is essentially planting a bomb onboard an aircraft. Except in this case the airline is buying the part (bomb) and installing it themselves.