Something that they thought existed but actually doesn't.
During our childhood days, when we used to pester our parents or elders to fulfill our demands. Or when we denied going to bed on time, our mother used to scare us by asking us to sleep on time or else spirits will arrive. But when we grew up, we realized that there's no such existence of spirits or ghosts in real.
Similarly, at some point in our lives, we do realize that we believed in some or the other things that don't actually exist.
Let's take a look at many such interesting but embarrassing confessions of people, who believed in things which never ever existed.
I thought that dogs were male and cats were female of the same animal.
For years as a child, my parents took phone calls from my Aunt Carol, received packages from her, and would go to the office to write her letters. I heard about her so much I formed an imaginary picture of this woman in my mind - the way she looked, talked, walked, etc. Turns out "Aunt Carol" was parent-code for weed. I have no Aunt Carol.
I used to think that everyone who had a bank account literally had a pigeon hole with their money deposited in it. And when someone wanted to take money out, the teller would take it from the pile of cash they had lodged over the years. It was a sad day finding out this wasn't the way it is.
The "Black Market." I literally thought there was a physical market you could go to to buy illegal items.
When I was young my father said to me: "Knowledge is Power - Francis Bacon." I understood it as "Knowledge is power, France is Bacon". For more than a decade I wondered over the meaning of the second part and what was the surreal linkage between the two? It wasn't until years later I saw it written down that the penny dropped.
Real life Power Rangers. Actually wondered what the hell they were doing when 9/11 happened.
I thought dinosaurs were still alive. I live by a mountain range, so I always thought that dinosaurs lived on the other side of the mountains.
When I was really little, I thought people lived in the TV.