This happened when I was in Kindergarten. I guess it was around St. Patrick's day, because the teacher was talking about leprachuns and such, when she said "Did you know, there's a pot of gold at the end of rainbows?" and this kid in the class said "I found the pot of gold at the end of the rainbow once, but a bird took it."
Well, the teacher didn't really say anything in response to the kid, so for years after that, I not only thought that the story of the pot of gold at the end of the rainbow was true, but also I thought that the pot of gold ended up in some bird's nest somewhere.
A childhood belief about birds

others believed things like this
- when I was a kid I used to believe that the end of the rainbow would be a pot of gold— jonas bryan proença de souza
- I used to believe that the rainbow is produced by the elves' pot of gold— Santiago Gordillo
- i believe that using gold give electric shock at the time of lightning.so i didnot wear gold when rain starts.— asi18
- I used to belive that there was gold in my nose because pepole would always say that i was "diggin' for gold"!— addicted nose picker
- Until I was 8 I used to believe that if clothing went into the wash more than once it would turn into gold.— Ben
- I used to believe the Golden Calf produced liquid gold instead of milk.— Trisha