Have you ever experienced a hypnopompic hallucination combined with sleep paralysis?
If you haven't, then you have no idea how real some of these experiences can be.
I had one in whcih I was waking up in my apartment, but I couldn't move. I could open my eyes, but all I could see was the corner of the ceiling above me. In my mind, I was in my bed back at my parents' house, and since most ceiling corners look the same, there was nothing to disabuse me of that hallucination.
The wall against which my headboard rested, in my parents' house, was between me and the stairs heading downstairs. I could hear a growling beastie, and I "knew" it was a large, demon-dog thing on the stairs. I was very scared, but the wrost part was that I could swear I heard my mom start walking up the stairs. I wanted, no, needed, to call out to her and get her to stop coming up, that the demon-dog would eat her. I was in a panic, but I couldn't move, couldn't make a sound.
Through sheer force of will, or perhaps because I was beginning to come out of this hypnopompic state, I was able to move my leg, and that sort of snapped it. I sat up quickly, saw my apartment bedroom, and it completely broke the spell.
I have to tell you, it was one of the most intense experiences I've ever had, and I can easily see how, had the dream been slightly different, I could have attributed it to aliens or a religious experience. Luckily, I had known about this phenomenon before, and could explain it to myself as soon as it was over.
So, dreams and other "misfirings" in the brain can cause very profound experiences that someone could easily believe had actually happened.