Zac S is spot on. Diagnosing mental disorder isn’t a precise science. Psychiatrists do their best based on what they observe, but as time passes they might decide that their diagnosis needs to be changed, or- very occasionally- accept that the person doesn’t actually have a mental disorder after all, but that the troubling behaviour they observed originated from something else e.g. an acute grief reaction.
However, even then it doesn’t mean that their original opinion can legally be challenged. Provided their diagnosis wasn’t unreasonable based on the evidence they had at the time, they will not be held legally liable for their ‘honest mistake’. Successive court cases from Bolam onwards have confirmed that they are legally protected if ‘responsible’ doctors could reasonably have come to the same conclusion.
Bear in mind that a section 2 detention requires two doctors’ recommendations not one, to reduce the risk of someone being detained based on one ‘rogue’ opinion, and at least one of them must be a qualified & approved psychiatrist, who is presumed to bring their experience to bear on what they see. In addition the AMHP making the application, which is underpinned by the two medical recommendations, will generally check that they contain enough information to justify making it.
Also, as Zac says, it’s very difficult to get someone admitted to psychiatric hospital nowadays because of the shortage of beds. No AMHP will go to the considerable trouble of making an application if they are not satisfied that it is needed because of the risk factors.
You can get your medical records to be amended in the light of later thinking- this happens a lot, though the original diagnosis usually stays on the record to reflect the history. But unless you can convincingly demonstrate to a court that the original diagnosis was so out of step with medical thinking that no responsible doctor could support it- either because they were clearly negligent in their approach, or they actually acted maliciously- no legal action against the doctors will succeed.
You can have your own view about all this. I know from talking to former patients how frustrating it is for those who think that they were wrongly diagnosed. But that is how the system works.