I have experienced the opposite. I once ordered from Ebay (I long ago stopped buying from Ebay) a Canon newFD lens. It was described as mint and photos showed too. It arrived loose in a plastic container, packed with padding in a cardboard box. Inside the plastic container were the lens and apart of it a snap-on front cap with protruding locking knobs. The filter-ring was badly cracked. Seemingly whilst sliding around in the container one knob was actuated and the cap went off, at a further bump at the cap the ring fractured.
Concerning materials: I think both plastic and metal have their advantages and disadvantages, We can see that at discussions on plastic versus metal top covers. At some point metal gets indented. Whereas plastic may be elastic enough to go back to the original state. In the worst case it may crack, but likely go back to original state. Of course one can argue what is worse. A indented cover may hamper a mechanical function.
So far I have come across many indented metal top covers, but not one fractured plastic one. Concerning fliter threads, a fractured plastic one still may function, whereas the slightest dent makes a metal one non-usable.
