Even in big airlines unexpected things can happen on CofA test flights after major maintenance.
And it doesn't have to involve test flights, just a plain old lack of knowledge and taking short cuts will achieve an unsatisfactory outcome. Many decades ago, I was in with one of the local Regulator's airworthiness PEs talking about whatever project. He, idly, mentioned a defect report which had just come across his desk regarding one of the two major airlines and the dispatch of a twin jet with one of the nacelle chine VGs missing. Apparently, the mx folk had deemed that missing component to be acceptable and had signed the aircraft to release pending spares. We had a rather urgent discussion on MELs, vortex flows, stall speeds, and asymmetric stall departures.
I never heard what the final outcome was but I suspect that there would have been some discussions over the road in the relevant mx hangar between tech services and mx ?