I've been misdiagnosed as bipolar in the past, and the medication regime I was put on lead to schizoid delusions and incredibly unstable emotional state that only resolved once I stopped taking the medication (under the monitoring of the psychiatrist who initially prescribed them).
My mother was also misdiagnosed as bipolar, leading to her actual condition of Borderline Personality Disorder going untreated until she attempted suicide (she died 6 months after because of complications relating to the attempt).
A friend of mine who is schizophrenic was also misdiagnosed as bipolar, because his delusions and paranoia were fairly minimal. In his case, the misdiagnosis was probably more helpful than not, as the medications were helpful in stabilising his mood.
My point here isn't that the medical establishment is bad or that doctors are stupid. It's that disorders related to mental illness are very, very difficult to diagnose and treat, even for incredibly intelligent trained professionals. Many disorders have significant overlap with other disorders. An effect whose impact is only made worse by the fact that a bunch of these disorders are highly co-morbid. Mix in the fact that the borders between healthy and unhealthy are not especially well defined, and it's easy to see where a lot of the contention comes from