I think a lot of people mistakenly believe "multiple personality disorder" (DID) is real because there are people who believe they have it and act accordingly.
There's a lot of evidence that cases for it are iatrogenic - that is to say, caused by the psychologist (and I suppose now, by social media). This lead to multiple lawsuits by patients who had spent years being convinced that they had DID and repressed memories (DID is usually linked to the also controversial idea of repressed memories by psychologists). See this New York Times article[1] from a few decades ago that talks about it more. You can Google "multiple personality disorder lawsuit" to see many more such cases. You see a lot of cases where bad psychology destroyed lives.
And the history of DID has always been _very_ questionable. If DID was a naturally occurring disorder, and one that's particularly noticeable, it's striking that (as far as I can tell) no one saw cases of it until very recently. Two cases and their dramatizations - that of Chris Costener Sizemore ("Eve") and Shirley Ardell Mason ("Sybil") - in particular where responsible for cementing it in the public's imagination, and leading to an enormous uptick in the diagnosis. If you look into those cases, you see that it's highly likely that they were iatrogenic - the patients weren't showing signs of DID, but he attention seeking psychiatrists "discovered" alternate personalities during their sessions using questionable techniques like hypnosis, and then used this to get rich and famous.
[1] https://www.nytimes.com/1997/11/06/us/memory-therapy-leads-t...