Hi' Being a work shop Manager, now retired, I have had similar problems with shipping packing ,blocks , spacers, and end caps being left in assemblies often causing major down time with serious damage to other components, the problem was the use of un trained labor hire mechanics/fitters, the company used to reduce the costs of employing and training their own labor.
I had, in an in your face position, in my office for upper management to see as they came in to my office when this sort of thing repeatedly happened a sticker with these words on it SKILLED LABOR ISN'T CHEAP, CHEAP LABOR ISN'T SKILLED. all I did was to point to the sticker and hand them the reports , we went from 26 full time fitter/mechanics/ welders/ to 5 over 20 years on a 24/7, 35 acre chemical plant that is now no more.