Just found out about two wild-sounding syndromes which people can get. One is called Capgras syndrome:

People with Capgras Syndrome act as if they are in a parallel universe in which the people they know are “doubles” or “impostors.”

They also frequently don’t recognize themselves in mirrors, or else think the person there is a doppleganger, and not actually them. (There’s a good article about it at the Pittsburgh Post Gazette). I’ve had that feeling happen to me before, for like a split second, while looking into the mirror. Wow, it’s also sometimes referred to as the “illusion of negative doubles” which is a really wild name. This other page about it says it frequently happens to paranoid schizophrenics. It sound very much like its something from a Philip K Dick novel. Another page has some more links on the subject.

The other wild syndrome is called Charles Bonnett Syndrome. In Bonnett Syndrome, people suffer from a particular type of eye disease, and then begin hallucinating otherworldly beings. These generally take the shape of ghosts & apparitions, as well as little costumed beings like elves or gnomes. But there is supposedly not any associated brain or psychiatric impairment. I’ve never seen any elves, although a friend told me about how her aunt saw what she called leprechauns before. A medical-looking site about Bonnett Syndrome. Plus one other article about it (and one more). Of course, none of the “serious” articles about either of these syndromes come even remotely close to suggesting that what people are actually seeing is somehow “real” or that they are experience perhaps parallel universes. Although, the articles which I originally found on that RealityCarnival site do indeed suggest that.