as a mechanic we’d find all kind of crap from the factory on small jets, tools left in the belly, FOD of all sizes, outflow valves incorrectly wired.
saw the inside of a bearing race closed off to the rest of the engine shot peened over the years due to a piece of tooling that had broken off during assembly and closed off without being noticed.
did a stint building comm satellites, there are all sorts of tools forever lodged inside those things flying through space. The once constant is that QC will never catch everything.