Some examples from my personal experience:
-When I was little, I believed that researchers were really fighting each other when I heard “researchers are fighting to find a cure for this disease”.
-I thought an autopilot actually worked like in the movie “Airplane”, that it was really an inflatable dummy.
In my early teens I believed and managed to convince a friend that Led Zeppelin was not a band, but a person with the first name Led and last name Zeppelin. We are not native speakers, so it made some sense…