Can multiple people hallucinate the same thing at the same time ?

[The Grand Nudger]

So how am i supposed to explain this ?
" When I was around ten years old, I lived in a house built in the 1700's. I was with my mom in my living room, when we both saw a kid around five years old, in old fashiond clothes, come out of the dining room and look around. My mom asked the kid if he was lost. He shook his head no, turned left, and walked through the hallway wall. That was 20 years ago. I remember it as clear as day, and it was like looking at a living person. I don't know if it was a ghost, some sort of time anomaly that showed me a kid from 300 years ago, or whatever the f, but both my mother and I remember it exactly the same to this day, down to the color of what he was wearing  "

Can anyone explain this ? Except that all these people are making it all up ?

All these people?  There are only two people in your narrative.  It's much easier to explain how a mother and child could come to a single narrative over twenty years than examples of actual mass hysteria or delusion. Since it seems like it will eventually come to a head and the bandaid may as well be ripped off..and because you absolutely insist on framing this as an issue of honesty. Yes, the mother and the child are lying to each other and to themselves. Or, conversely, there is only one dishonest narrator involved - the person who told the story to you.