One particularly interesting element of being diagnosed with a personality disorder is the way that people who believe the label perceive and treat you.
It's sometimes difficult for me to get people to understand that autism or ADHD leads to certain kinds of behavior, and how to accommodate it. It's very easy for people who know I was previously diagnosed with BPD and believe it's true to characterize everything I say and do as symptomatic of BPD, because "symptomatic of BPD" is a fairly large and amorphous category. Typical, everyday behaviors that were acceptable become "manipulative" and "passive aggressive" and if I get frustrated or angry about something, it's because I'm an oversensitive borderline and couldn't possibly have a legitimate complaint. Fortunately, I encountered very little of this due to the fact that my family, friends, and therapist don't believe the diagnosis was true. But I have encountered some. Some on this forum, some on another forum (all but one person involved on that other forum has since apologized and admitted they were wrong, and we're actually still on good terms).
The paper I linked earlier (I think on page 3) talks about how reasonable behavior from people diagnosed with BPD are reframed as hostile and manipulative and deconstructs these perceptions to show how they're not just false but harmful.
Another insidious element of being misdiagnosed with a personality disorder is that people who know just enough about them to be dangerous know that often people with PDs tend to have a lot of defense mechanisms to rationalize, justify, ignore, or deny their own behavior as problematic. Everyone else is the problem, they're just fine. This is called "egosyntonic" which means you consider these traits to be a normal and okay part of your personality. This is contrasted with "egodystonic" where the traits clash with your sense of self and who you want to be.
So, in practical terms, this means that people diagnosed with certain PDs especially are not just expected to deny it, but also to resist treatment. This means that saying you do not have a personality disorder, or the traits of a personality disorder then becomes symptomatic of that personality disorder. Denial in essence becomes proof to those who already want to believe you really have a PD.
When I saw my therapist the first time, I told her I had been diagnosed with BPD, told her I didn't believe it was true, and told her that I was aware that often people diagnosed with PDs deny it. So I asked her explicitly to let me know if I displayed any BPD-like behavior, as she's worked with a lot of people who have been diagnosed with BPD. After 10 months, her assessment was that I was not like anyone else she's ever worked with, and especially not like those diagnosed with BPD. I didn't offer her rationalizations for my behavior, or go down my list of reasons why I don't believe I have BPD. I just asked her to look for signs and talked about my problems.
And I wasn't really against the idea of having it - if I really have it, I really want to know that I have it, because I have a lifetime of difficulties to deal with, and I want to map them out and understand them as thoroughly as possible. It would be completely irrational to simply reject out of hand a particular diagnosis because I didn't like it. But if I don't actually show the signs of that diagnosis, I do not want to be diagnosed with it. I doubt most people want that.
There was the additional issue when I was diagnosed that I was worried that the diagnosis would sabotage my chance at therapy as well as my medical care. This has not yet happened, thankfully. Really, medical professionals in my local area are some of the best I've personally dealt with, at least in terms of listening to their clients and helping. None of them are perfect - I could go on all day how my therapist misunderstands some of the things I've said and bulldozes over my explanations until I lose track of the conversation - but they're better than many I've had.