When my 200-500 arrived I went out to photograph some critters in the neighborhood and found everything was extremely soft - not just a little, but clearly not at all sharp. I started testing, put out some staggered containers to check dof, fine tuning, etc. I couldn't get things sharp. It was my first telephoto and I knew there was am added skillset with these lenses so I wondered it it was my fault.
I went on a photography forum and asked for advice, sharing mantmy sample images including the DoF tests. Most people blamed me, said it was my hand holding (even when on a tripod) my poor technique, everything.
Eventually, acting against the advice of most of the people on that forum, I sent it back. It took about a month to get a new copy, but when it arrived and I took the first shot I was instantly certain the other lens had been defective. Saying the difference was night and day would be a vast understatement.
I see similar things all the time on forums and it frustrates me greatly. Anytime anyone asks about or reports a problem with equipment, many people flock to tell them it's their fault, that they've got bad technique, that it's user error, that equipment defects are very rare, etc. Keep in mind that with my experience the 200-500 was known to have had significant problems with bad copies for some production runs and still people collectively assumed first that I was the problem.
That doesn't mean it's never a user's error. Sometimes it is, but to I wish people would be a heck of a lot more open to the possibility it's not.