Particle contamination is a very common problem with airguns. It's one of the most common causes of valve leaks causing air to leak out the barrel. It can also cause regulators to fail. All components must be carefully cleaned and checked during assembly and before installation to make sure they are free of filings, grit and other potentially damaging particles.
In the photos above, you can see a small gold-colored piece of sintered filter embedded into the face of a valve stem. The sintered filter itself can be seen in the fill nipple of the regulator shown in the photo on the right. I assume the piece of filter was behind the pin valve on the regulator, then blew into the gun when the tank and regulator were threaded into the tank block. It was on it's way through the valve when the stem closed on it. This not only damaged the stem but the Magnum Valve which had a deep pit in the mating surface. Both parts needed to be replaced. It was a costly repair for the customer who put a contaminated regulator on the custom HPA gun he bought from me. Never assume a part or component is clean.
I've seen filter particles caught in valve stems many times. Metal filings left over from machining, drilling and grit from abrasives are even more common. Every component of the guns and parts I sell are thoroughly cleaned and checked to prevent contamination. Even so, parts should always be checked before assembly to make sure they didn't become contaminated somewhere along the way.