Touching on the intersection of crazy people and technology:
Recall that the larger portion of what we experience as vision is generated from internal stimulus; it is hallucinated to fill in the blanks in the actual perceptual stimuli we receive.
Now imagine that what a person experiences as vision is projected back into the world, maybe they've got a brain interface that reads everything coming out of the visual cortex and transmits it outside the body. The effect (for the purposes of this imaginary short-story) would be that people with similar brain-interfaces could receive the output of others' visual experience and add it to their own; in effect overlaying other people's perception with their own. Just imagine that's how it works for the moment.
So: if a person who experienced vivid hallucinations were thrown into the mix, those hallucinations would be shared with everyone they interacted with. They could imagine a person, interact with it, and treat it like an autonomous human being (I'm thinking "Beautiful Mind" here), being unable to tell the hallucination from reality. The catch is that other people would also be unable to distinguish the hallucinations' unreality; and they would treat the others as being real.
Here's a disturbing question: if everyone agrees these imagined people are real, are they?