Hey, there's folks who are trained armorers ... and then there's folks who are trained armorers ...
I've yet to go to an armorer class in which I'd let 95% of the class touch one of my weapons.
Granted, I made my fair share of mistakes after returning from my first couple of armorer classes, but I was fortunate in that I was able to basically apprentice myself to a much more experienced armorer at my agency. Even after more than a dozen armorer classes I was still learning new things about various platforms. The more classes I attended, though, the more stories of problems I heard from other armorers.
I've seen junior armorers I've tried to assist do their own fair share of mixing up parts, installing parts backwards or in some incorrect orientation that I hadn't seen done up to that point.
As with other training venues, sometimes a newly minted armorer returning from their first armorer class may have learned just enough to really cause some problems.
I've seen armorers have problems in recert classes, too.
FWIW, I try to make sure the cotton-tip swabs I use resist coming apart easily ... and I check for errant fibers left behind, too.