I've seen a lot of strange diagnoses which made me realise how little knowlegde some mental health professionals have about certain problems.

I've seen people...
With PTSD having a borderline diagnose.
Who dealt with aggressive/rejecting behavior with a diagnose depression.
Who dealt with manipulative, abusive behavior having a diagnose autism, depression, anxiety disorder and being recommended to eat more of specific kinds of food.
Who had a narcissistic partner being diagnosed as narcissistic (possibly because of the projections that were seen as well thought feedback).

I think arrogance can sometimes also contribute to people making wrong diagnoses, possibly thinking because of their knowlegde, that they don't need much feedback from the person they are helping.
People who gave me suitable support were often people who listened well, thought about what I was saying and asked questions before creating conclusions, the conclusions were then used for further understanding in a way that what I thought about things contributed to a better understanding both for me and the person that was supporting me.