I know of a certain home built light aircraft where the maximum advertised engine rpm couldn't be achieved during engine break-in.
The engine manufacturer's rep was initially fooled, but eventually discovered that a "pusher" prop had been supplied and incorrectly fitted to the "tractor" engine by mistake....
After that slight issue had been resolved, the same aircraft crashed on its second flight, because the supposed "belt driven" reduction drive box wasn't actually belt driven - it was a gearbox and didn't have any gear oil in it.... despite the clear warnings in the build manual. I was more than a little glad that I'd declined the builder's request to test fly that particular aircraft.